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nep-0004-datetime-proposal3.html
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<section id="nep-4-a-third-proposal-for-implementing-some-date-time-types-in-numpy">
<span id="nep04"></span><h1>NEP 4 — A (third) proposal for implementing some date/time types in NumPy<a class="headerlink" href="#nep-4-a-third-proposal-for-implementing-some-date-time-types-in-numpy" title="Link to this heading">#</a></h1>
<dl class="field-list simple">
<dt class="field-odd">Author<span class="colon">:</span></dt>
<dd class="field-odd"><p>Francesc Alted i Abad</p>
</dd>
<dt class="field-even">Contact<span class="colon">:</span></dt>
<dd class="field-even"><p><a class="reference external" href="mailto:faltet%40pytables.com">faltet<span>@</span>pytables<span>.</span>com</a></p>
</dd>
<dt class="field-odd">Author<span class="colon">:</span></dt>
<dd class="field-odd"><p>Ivan Vilata i Balaguer</p>
</dd>
<dt class="field-even">Contact<span class="colon">:</span></dt>
<dd class="field-even"><p><a class="reference external" href="mailto:ivan%40selidor.net">ivan<span>@</span>selidor<span>.</span>net</a></p>
</dd>
<dt class="field-odd">Date<span class="colon">:</span></dt>
<dd class="field-odd"><p>2008-07-30</p>
</dd>
<dt class="field-even">Status<span class="colon">:</span></dt>
<dd class="field-even"><p>Deferred</p>
</dd>
</dl>
<section id="executive-summary">
<h2>Executive summary<a class="headerlink" href="#executive-summary" title="Link to this heading">#</a></h2>
<p>A date/time mark is something very handy to have in many fields where
one has to deal with data sets. While Python has several modules that
define a date/time type (like the integrated <code class="docutils literal notranslate"><span class="pre">datetime</span></code> <a class="footnote-reference brackets" href="#id10" id="id1" role="doc-noteref"><span class="fn-bracket">[</span>1<span class="fn-bracket">]</span></a> or
<code class="docutils literal notranslate"><span class="pre">mx.DateTime</span></code> <a class="footnote-reference brackets" href="#id11" id="id2" role="doc-noteref"><span class="fn-bracket">[</span>2<span class="fn-bracket">]</span></a>), NumPy has a lack of them.</p>
<p>In this document, we are proposing the addition of a series of date/time
types to fill this gap. The requirements for the proposed types are
two-folded: 1) they have to be fast to operate with and 2) they have to
be as compatible as possible with the existing <code class="docutils literal notranslate"><span class="pre">datetime</span></code> module that
comes with Python.</p>
</section>
<section id="types-proposed">
<h2>Types proposed<a class="headerlink" href="#types-proposed" title="Link to this heading">#</a></h2>
<p>To start with, it is virtually impossible to come up with a single
date/time type that fills the needs of every case of use. So, after
pondering about different possibilities, we have stuck with <em>two</em>
different types, namely <code class="docutils literal notranslate"><span class="pre">datetime64</span></code> and <code class="docutils literal notranslate"><span class="pre">timedelta64</span></code> (these names
are preliminary and can be changed), that can have different time units
so as to cover different needs.</p>
<div class="admonition important">
<p class="admonition-title">Important</p>
<p>the time unit is conceived here as metadata that
<em>complements</em> a date/time dtype, <em>without changing the base type</em>. It
provides information about the <em>meaning</em> of the stored numbers, not
about their <em>structure</em>.</p>
</div>
<p>Now follows a detailed description of the proposed types.</p>
<section id="datetime64">
<h3><code class="docutils literal notranslate"><span class="pre">datetime64</span></code><a class="headerlink" href="#datetime64" title="Link to this heading">#</a></h3>
<p>It represents a time that is absolute (i.e. not relative). It is
implemented internally as an <code class="docutils literal notranslate"><span class="pre">int64</span></code> type. The internal epoch is the
POSIX epoch (see <a class="footnote-reference brackets" href="#id12" id="id3" role="doc-noteref"><span class="fn-bracket">[</span>3<span class="fn-bracket">]</span></a>). Like POSIX, the representation of a date
doesn’t take leap seconds into account.</p>
<p>In time unit <em>conversions</em> and time <em>representations</em> (but not in other
time computations), the value -2**63 (0x8000000000000000) is interpreted
as an invalid or unknown date, <em>Not a Time</em> or <em>NaT</em>. See the section
on time unit conversions for more information.</p>
<section id="time-units">
<h4>Time units<a class="headerlink" href="#time-units" title="Link to this heading">#</a></h4>
<p>It accepts different time units, each of them implying a different time
span. The table below describes the time units supported with their
corresponding time spans.</p>
<table class="table">
<thead>
<tr class="row-odd"><th class="head" colspan="2"><p>Time unit</p></th>
<th class="head"><p>Time span (years)</p></th>
</tr>
<tr class="row-even"><th class="head"><p>Code</p></th>
<th class="head"><p>Meaning</p></th>
<th class="head"></th>
</tr>
</thead>
<tbody>
<tr class="row-odd"><td><p>Y</p></td>
<td><p>year</p></td>
<td><p>[9.2e18 BC, 9.2e18 AD]</p></td>
</tr>
<tr class="row-even"><td><p>M</p></td>
<td><p>month</p></td>
<td><p>[7.6e17 BC, 7.6e17 AD]</p></td>
</tr>
<tr class="row-odd"><td><p>W</p></td>
<td><p>week</p></td>
<td><p>[1.7e17 BC, 1.7e17 AD]</p></td>
</tr>
<tr class="row-even"><td><p>B</p></td>
<td><p>business day</p></td>
<td><p>[3.5e16 BC, 3.5e16 AD]</p></td>
</tr>
<tr class="row-odd"><td><p>D</p></td>
<td><p>day</p></td>
<td><p>[2.5e16 BC, 2.5e16 AD]</p></td>
</tr>
<tr class="row-even"><td><p>h</p></td>
<td><p>hour</p></td>
<td><p>[1.0e15 BC, 1.0e15 AD]</p></td>
</tr>
<tr class="row-odd"><td><p>m</p></td>
<td><p>minute</p></td>
<td><p>[1.7e13 BC, 1.7e13 AD]</p></td>
</tr>
<tr class="row-even"><td><p>s</p></td>
<td><p>second</p></td>
<td><p>[ 2.9e9 BC, 2.9e9 AD]</p></td>
</tr>
<tr class="row-odd"><td><p>ms</p></td>
<td><p>millisecond</p></td>
<td><p>[ 2.9e6 BC, 2.9e6 AD]</p></td>
</tr>
<tr class="row-even"><td><p>us</p></td>
<td><p>microsecond</p></td>
<td><p>[290301 BC, 294241 AD]</p></td>
</tr>
<tr class="row-odd"><td><p>c#</p></td>
<td><p>ticks (100ns)</p></td>
<td><p>[ 2757 BC, 31197 AD]</p></td>
</tr>
<tr class="row-even"><td><p>ns</p></td>
<td><p>nanosecond</p></td>
<td><p>[ 1678 AD, 2262 AD]</p></td>
</tr>
</tbody>
</table>
<p>The value of an absolute date is thus <em>an integer number of units of the
chosen time unit</em> passed since the internal epoch. When working with
business days, Saturdays and Sundays are simply ignored from the count
(i.e. day 3 in business days is not Saturday 1970-01-03, but Monday
1970-01-05).</p>
</section>
<section id="building-a-datetime64-dtype">
<h4>Building a <code class="docutils literal notranslate"><span class="pre">datetime64</span></code> dtype<a class="headerlink" href="#building-a-datetime64-dtype" title="Link to this heading">#</a></h4>
<p>The proposed ways to specify the time unit in the dtype constructor are:</p>
<p>Using the long string notation:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">dtype</span><span class="p">(</span><span class="s1">'datetime64[us]'</span><span class="p">)</span>
</pre></div>
</div>
<p>Using the short string notation:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">dtype</span><span class="p">(</span><span class="s1">'M8[us]'</span><span class="p">)</span>
</pre></div>
</div>
<p>The default is microseconds if no time unit is specified. Thus, ‘M8’ is equivalent to ‘M8[us]’</p>
</section>
<section id="setting-and-getting-values">
<h4>Setting and getting values<a class="headerlink" href="#setting-and-getting-values" title="Link to this heading">#</a></h4>
<p>The objects with this dtype can be set in a series of ways:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">t</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'M8[s]'</span><span class="p">)</span>
<span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1199164176</span> <span class="c1"># assign to July 30th, 2008 at 17:31:00</span>
<span class="n">t</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="n">datetime</span><span class="o">.</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2008</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">30</span><span class="p">,</span> <span class="mi">17</span><span class="p">,</span> <span class="mi">31</span><span class="p">,</span> <span class="mi">01</span><span class="p">)</span> <span class="c1"># with datetime module</span>
<span class="n">t</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'2008-07-30T17:31:02'</span> <span class="c1"># with ISO 8601</span>
</pre></div>
</div>
<p>And can be get in different ways too:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">str</span><span class="p">(</span><span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span> <span class="o">--></span> <span class="mi">2008</span><span class="o">-</span><span class="mi">07</span><span class="o">-</span><span class="mi">30</span><span class="n">T17</span><span class="p">:</span><span class="mi">31</span><span class="p">:</span><span class="mi">00</span>
<span class="nb">repr</span><span class="p">(</span><span class="n">t</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span> <span class="o">--></span> <span class="n">datetime64</span><span class="p">(</span><span class="mi">1199164177</span><span class="p">,</span> <span class="s1">'s'</span><span class="p">)</span>
<span class="nb">str</span><span class="p">(</span><span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">item</span><span class="p">())</span> <span class="o">--></span> <span class="mi">2008</span><span class="o">-</span><span class="mi">07</span><span class="o">-</span><span class="mi">30</span> <span class="mi">17</span><span class="p">:</span><span class="mi">31</span><span class="p">:</span><span class="mi">00</span> <span class="c1"># datetime module object</span>
<span class="nb">repr</span><span class="p">(</span><span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">item</span><span class="p">())</span> <span class="o">--></span> <span class="n">datetime</span><span class="o">.</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2008</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">30</span><span class="p">,</span> <span class="mi">17</span><span class="p">,</span> <span class="mi">31</span><span class="p">)</span> <span class="c1"># idem</span>
<span class="nb">str</span><span class="p">(</span><span class="n">t</span><span class="p">)</span> <span class="o">--></span> <span class="p">[</span><span class="mi">2008</span><span class="o">-</span><span class="mi">07</span><span class="o">-</span><span class="mi">30</span><span class="n">T17</span><span class="p">:</span><span class="mi">31</span><span class="p">:</span><span class="mi">00</span> <span class="mi">2008</span><span class="o">-</span><span class="mi">07</span><span class="o">-</span><span class="mi">30</span><span class="n">T17</span><span class="p">:</span><span class="mi">31</span><span class="p">:</span><span class="mi">01</span> <span class="mi">2008</span><span class="o">-</span><span class="mi">07</span><span class="o">-</span><span class="mi">30</span><span class="n">T17</span><span class="p">:</span><span class="mi">31</span><span class="p">:</span><span class="mi">02</span><span class="p">]</span>
<span class="nb">repr</span><span class="p">(</span><span class="n">t</span><span class="p">)</span> <span class="o">--></span> <span class="n">array</span><span class="p">([</span><span class="mi">1199164176</span><span class="p">,</span> <span class="mi">1199164177</span><span class="p">,</span> <span class="mi">1199164178</span><span class="p">],</span>
<span class="n">dtype</span><span class="o">=</span><span class="s1">'datetime64[s]'</span><span class="p">)</span>
</pre></div>
</div>
</section>
<section id="comparisons">
<h4>Comparisons<a class="headerlink" href="#comparisons" title="Link to this heading">#</a></h4>
<p>The comparisons will be supported too:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="s1">'1980'</span><span class="p">],</span> <span class="s1">'M8[Y]'</span><span class="p">)</span> <span class="o">==</span> <span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="s1">'1979'</span><span class="p">],</span> <span class="s1">'M8[Y]'</span><span class="p">)</span>
<span class="o">--></span> <span class="p">[</span><span class="kc">False</span><span class="p">]</span>
</pre></div>
</div>
<p>or by applying broadcasting:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="s1">'1979'</span><span class="p">,</span> <span class="s1">'1980'</span><span class="p">],</span> <span class="s1">'M8[Y]'</span><span class="p">)</span> <span class="o">==</span> <span class="n">numpy</span><span class="o">.</span><span class="n">datetime64</span><span class="p">(</span><span class="s1">'1980'</span><span class="p">,</span> <span class="s1">'Y'</span><span class="p">)</span>
<span class="o">--></span> <span class="p">[</span><span class="kc">False</span><span class="p">,</span> <span class="kc">True</span><span class="p">]</span>
</pre></div>
</div>
<p>The next should work too:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="s1">'1979'</span><span class="p">,</span> <span class="s1">'1980'</span><span class="p">],</span> <span class="s1">'M8[Y]'</span><span class="p">)</span> <span class="o">==</span> <span class="s1">'1980-01-01'</span>
<span class="o">--></span> <span class="p">[</span><span class="kc">False</span><span class="p">,</span> <span class="kc">True</span><span class="p">]</span>
</pre></div>
</div>
<p>because the right hand expression can be broadcasted into an array of 2
elements of dtype ‘M8[Y]’.</p>
</section>
<section id="compatibility-issues">
<h4>Compatibility issues<a class="headerlink" href="#compatibility-issues" title="Link to this heading">#</a></h4>
<p>This will be fully compatible with the <code class="docutils literal notranslate"><span class="pre">datetime</span></code> class of the
<code class="docutils literal notranslate"><span class="pre">datetime</span></code> module of Python only when using a time unit of
microseconds. For other time units, the conversion process will lose
precision or will overflow as needed. The conversion from/to a
<code class="docutils literal notranslate"><span class="pre">datetime</span></code> object doesn’t take leap seconds into account.</p>
</section>
</section>
<section id="timedelta64">
<h3><code class="docutils literal notranslate"><span class="pre">timedelta64</span></code><a class="headerlink" href="#timedelta64" title="Link to this heading">#</a></h3>
<p>It represents a time that is relative (i.e. not absolute). It is
implemented internally as an <code class="docutils literal notranslate"><span class="pre">int64</span></code> type.</p>
<p>In time unit <em>conversions</em> and time <em>representations</em> (but not in other
time computations), the value -2**63 (0x8000000000000000) is interpreted
as an invalid or unknown time, <em>Not a Time</em> or <em>NaT</em>. See the section
on time unit conversions for more information.</p>
<section id="id4">
<h4>Time units<a class="headerlink" href="#id4" title="Link to this heading">#</a></h4>
<p>It accepts different time units, each of them implying a different time
span. The table below describes the time units supported with their
corresponding time spans.</p>
<table class="table">
<thead>
<tr class="row-odd"><th class="head" colspan="2"><p>Time unit</p></th>
<th class="head"><p>Time span</p></th>
</tr>
<tr class="row-even"><th class="head"><p>Code</p></th>
<th class="head"><p>Meaning</p></th>
<th class="head"></th>
</tr>
</thead>
<tbody>
<tr class="row-odd"><td><p>Y</p></td>
<td><p>year</p></td>
<td><p>+- 9.2e18 years</p></td>
</tr>
<tr class="row-even"><td><p>M</p></td>
<td><p>month</p></td>
<td><p>+- 7.6e17 years</p></td>
</tr>
<tr class="row-odd"><td><p>W</p></td>
<td><p>week</p></td>
<td><p>+- 1.7e17 years</p></td>
</tr>
<tr class="row-even"><td><p>B</p></td>
<td><p>business day</p></td>
<td><p>+- 3.5e16 years</p></td>
</tr>
<tr class="row-odd"><td><p>D</p></td>
<td><p>day</p></td>
<td><p>+- 2.5e16 years</p></td>
</tr>
<tr class="row-even"><td><p>h</p></td>
<td><p>hour</p></td>
<td><p>+- 1.0e15 years</p></td>
</tr>
<tr class="row-odd"><td><p>m</p></td>
<td><p>minute</p></td>
<td><p>+- 1.7e13 years</p></td>
</tr>
<tr class="row-even"><td><p>s</p></td>
<td><p>second</p></td>
<td><p>+- 2.9e12 years</p></td>
</tr>
<tr class="row-odd"><td><p>ms</p></td>
<td><p>millisecond</p></td>
<td><p>+- 2.9e9 years</p></td>
</tr>
<tr class="row-even"><td><p>us</p></td>
<td><p>microsecond</p></td>
<td><p>+- 2.9e6 years</p></td>
</tr>
<tr class="row-odd"><td><p>c#</p></td>
<td><p>ticks (100ns)</p></td>
<td><p>+- 2.9e4 years</p></td>
</tr>
<tr class="row-even"><td><p>ns</p></td>
<td><p>nanosecond</p></td>
<td><p>+- 292 years</p></td>
</tr>
<tr class="row-odd"><td><p>ps</p></td>
<td><p>picosecond</p></td>
<td><p>+- 106 days</p></td>
</tr>
<tr class="row-even"><td><p>fs</p></td>
<td><p>femtosecond</p></td>
<td><p>+- 2.6 hours</p></td>
</tr>
<tr class="row-odd"><td><p>as</p></td>
<td><p>attosecond</p></td>
<td><p>+- 9.2 seconds</p></td>
</tr>
</tbody>
</table>
<p>The value of a time delta is thus <em>an integer number of units of the
chosen time unit</em>.</p>
</section>
<section id="building-a-timedelta64-dtype">
<h4>Building a <code class="docutils literal notranslate"><span class="pre">timedelta64</span></code> dtype<a class="headerlink" href="#building-a-timedelta64-dtype" title="Link to this heading">#</a></h4>
<p>The proposed ways to specify the time unit in the dtype constructor are:</p>
<p>Using the long string notation:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">dtype</span><span class="p">(</span><span class="s1">'timedelta64[us]'</span><span class="p">)</span>
</pre></div>
</div>
<p>Using the short string notation:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">dtype</span><span class="p">(</span><span class="s1">'m8[us]'</span><span class="p">)</span>
</pre></div>
</div>
<p>The default is micro-seconds if no default is specified: ‘m8’ is equivalent to ‘m8[us]’</p>
</section>
<section id="id5">
<h4>Setting and getting values<a class="headerlink" href="#id5" title="Link to this heading">#</a></h4>
<p>The objects with this dtype can be set in a series of ways:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">t</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'m8[ms]'</span><span class="p">)</span>
<span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">12</span> <span class="c1"># assign to 12 ms</span>
<span class="n">t</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="n">datetime</span><span class="o">.</span><span class="n">timedelta</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">13000</span><span class="p">)</span> <span class="c1"># 13 ms</span>
<span class="n">t</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'0:00:00.014'</span> <span class="c1"># 14 ms</span>
</pre></div>
</div>
<p>And can be get in different ways too:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">str</span><span class="p">(</span><span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span> <span class="o">--></span> <span class="mi">0</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">00.012</span>
<span class="nb">repr</span><span class="p">(</span><span class="n">t</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span> <span class="o">--></span> <span class="n">timedelta64</span><span class="p">(</span><span class="mi">13</span><span class="p">,</span> <span class="s1">'ms'</span><span class="p">)</span>
<span class="nb">str</span><span class="p">(</span><span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">item</span><span class="p">())</span> <span class="o">--></span> <span class="mi">0</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">00.012000</span> <span class="c1"># datetime module object</span>
<span class="nb">repr</span><span class="p">(</span><span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">item</span><span class="p">())</span> <span class="o">--></span> <span class="n">datetime</span><span class="o">.</span><span class="n">timedelta</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">12000</span><span class="p">)</span> <span class="c1"># idem</span>
<span class="nb">str</span><span class="p">(</span><span class="n">t</span><span class="p">)</span> <span class="o">--></span> <span class="p">[</span><span class="mi">0</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">00.012</span> <span class="mi">0</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">00.014</span> <span class="mi">0</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">00.014</span><span class="p">]</span>
<span class="nb">repr</span><span class="p">(</span><span class="n">t</span><span class="p">)</span> <span class="o">--></span> <span class="n">array</span><span class="p">([</span><span class="mi">12</span><span class="p">,</span> <span class="mi">13</span><span class="p">,</span> <span class="mi">14</span><span class="p">],</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"timedelta64[ms]"</span><span class="p">)</span>
</pre></div>
</div>
</section>
<section id="id6">
<h4>Comparisons<a class="headerlink" href="#id6" title="Link to this heading">#</a></h4>
<p>The comparisons will be supported too:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="mi">12</span><span class="p">,</span> <span class="mi">13</span><span class="p">,</span> <span class="mi">14</span><span class="p">],</span> <span class="s1">'m8[ms]'</span><span class="p">)</span> <span class="o">==</span> <span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="mi">12</span><span class="p">,</span> <span class="mi">13</span><span class="p">,</span> <span class="mi">13</span><span class="p">],</span> <span class="s1">'m8[ms]'</span><span class="p">)</span>
<span class="o">--></span> <span class="p">[</span><span class="kc">True</span><span class="p">,</span> <span class="kc">True</span><span class="p">,</span> <span class="kc">False</span><span class="p">]</span>
</pre></div>
</div>
<p>or by applying broadcasting:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="mi">12</span><span class="p">,</span> <span class="mi">13</span><span class="p">,</span> <span class="mi">14</span><span class="p">],</span> <span class="s1">'m8[ms]'</span><span class="p">)</span> <span class="o">==</span> <span class="n">numpy</span><span class="o">.</span><span class="n">timedelta64</span><span class="p">(</span><span class="mi">13</span><span class="p">,</span> <span class="s1">'ms'</span><span class="p">)</span>
<span class="o">--></span> <span class="p">[</span><span class="kc">False</span><span class="p">,</span> <span class="kc">True</span><span class="p">,</span> <span class="kc">False</span><span class="p">]</span>
</pre></div>
</div>
<p>The next should work too:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="mi">12</span><span class="p">,</span> <span class="mi">13</span><span class="p">,</span> <span class="mi">14</span><span class="p">],</span> <span class="s1">'m8[ms]'</span><span class="p">)</span> <span class="o">==</span> <span class="s1">'0:00:00.012'</span>
<span class="o">--></span> <span class="p">[</span><span class="kc">True</span><span class="p">,</span> <span class="kc">False</span><span class="p">,</span> <span class="kc">False</span><span class="p">]</span>
</pre></div>
</div>
<p>because the right hand expression can be broadcasted into an array of 3
elements of dtype ‘m8[ms]’.</p>
</section>
<section id="id7">
<h4>Compatibility issues<a class="headerlink" href="#id7" title="Link to this heading">#</a></h4>
<p>This will be fully compatible with the <code class="docutils literal notranslate"><span class="pre">timedelta</span></code> class of the
<code class="docutils literal notranslate"><span class="pre">datetime</span></code> module of Python only when using a time unit of
microseconds. For other units, the conversion process will lose
precision or will overflow as needed.</p>
</section>
</section>
</section>
<section id="examples-of-use">
<h2>Examples of use<a class="headerlink" href="#examples-of-use" title="Link to this heading">#</a></h2>
<p>Here it is an example of use for the <code class="docutils literal notranslate"><span class="pre">datetime64</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">5</span><span class="p">]:</span> <span class="n">numpy</span><span class="o">.</span><span class="n">datetime64</span><span class="p">(</span><span class="mi">42</span><span class="p">,</span> <span class="s1">'us'</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">5</span><span class="p">]:</span> <span class="n">datetime64</span><span class="p">(</span><span class="mi">42</span><span class="p">,</span> <span class="s1">'us'</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">6</span><span class="p">]:</span> <span class="nb">print</span> <span class="n">numpy</span><span class="o">.</span><span class="n">datetime64</span><span class="p">(</span><span class="mi">42</span><span class="p">,</span> <span class="s1">'us'</span><span class="p">)</span>
<span class="mi">1970</span><span class="o">-</span><span class="mi">01</span><span class="o">-</span><span class="mi">01</span><span class="n">T00</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">00.000042</span> <span class="c1"># representation in ISO 8601 format</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">7</span><span class="p">]:</span> <span class="nb">print</span> <span class="n">numpy</span><span class="o">.</span><span class="n">datetime64</span><span class="p">(</span><span class="mf">367.7</span><span class="p">,</span> <span class="s1">'D'</span><span class="p">)</span> <span class="c1"># decimal part is lost</span>
<span class="mi">1971</span><span class="o">-</span><span class="mi">01</span><span class="o">-</span><span class="mi">02</span> <span class="c1"># still ISO 8601 format</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">8</span><span class="p">]:</span> <span class="n">numpy</span><span class="o">.</span><span class="n">datetime</span><span class="p">(</span><span class="s1">'2008-07-18T12:23:18'</span><span class="p">,</span> <span class="s1">'m'</span><span class="p">)</span> <span class="c1"># from ISO 8601</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">8</span><span class="p">]:</span> <span class="n">datetime64</span><span class="p">(</span><span class="mi">20273063</span><span class="p">,</span> <span class="s1">'m'</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">9</span><span class="p">]:</span> <span class="nb">print</span> <span class="n">numpy</span><span class="o">.</span><span class="n">datetime</span><span class="p">(</span><span class="s1">'2008-07-18T12:23:18'</span><span class="p">,</span> <span class="s1">'m'</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">9</span><span class="p">]:</span> <span class="mi">2008</span><span class="o">-</span><span class="mi">07</span><span class="o">-</span><span class="mi">18</span><span class="n">T12</span><span class="p">:</span><span class="mi">23</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">10</span><span class="p">]:</span> <span class="n">t</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">zeros</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"datetime64[ms]"</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">11</span><span class="p">]:</span> <span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="n">datetime</span><span class="o">.</span><span class="n">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">()</span> <span class="c1"># setter in action</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">12</span><span class="p">]:</span> <span class="nb">print</span> <span class="n">t</span>
<span class="p">[</span><span class="mi">2008</span><span class="o">-</span><span class="mi">07</span><span class="o">-</span><span class="mi">16</span><span class="n">T13</span><span class="p">:</span><span class="mi">39</span><span class="p">:</span><span class="mf">25.315</span> <span class="mi">1970</span><span class="o">-</span><span class="mi">01</span><span class="o">-</span><span class="mi">01</span><span class="n">T00</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">00.000</span>
<span class="mi">1970</span><span class="o">-</span><span class="mi">01</span><span class="o">-</span><span class="mi">01</span><span class="n">T00</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">00.000</span> <span class="mi">1970</span><span class="o">-</span><span class="mi">01</span><span class="o">-</span><span class="mi">01</span><span class="n">T00</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">00.000</span>
<span class="mi">1970</span><span class="o">-</span><span class="mi">01</span><span class="o">-</span><span class="mi">01</span><span class="n">T00</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">00.000</span><span class="p">]</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">13</span><span class="p">]:</span> <span class="nb">repr</span><span class="p">(</span><span class="n">t</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">13</span><span class="p">]:</span> <span class="n">array</span><span class="p">([</span><span class="mi">267859210457</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">],</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"datetime64[ms]"</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">14</span><span class="p">]:</span> <span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">item</span><span class="p">()</span> <span class="c1"># getter in action</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">14</span><span class="p">]:</span> <span class="n">datetime</span><span class="o">.</span><span class="n">datetime</span><span class="p">(</span><span class="mi">2008</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">16</span><span class="p">,</span> <span class="mi">13</span><span class="p">,</span> <span class="mi">39</span><span class="p">,</span> <span class="mi">25</span><span class="p">,</span> <span class="mi">315000</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">15</span><span class="p">]:</span> <span class="nb">print</span> <span class="n">t</span><span class="o">.</span><span class="n">dtype</span>
<span class="n">dtype</span><span class="p">(</span><span class="s1">'datetime64[ms]'</span><span class="p">)</span>
</pre></div>
</div>
<p>And here it goes an example of use for the <code class="docutils literal notranslate"><span class="pre">timedelta64</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">5</span><span class="p">]:</span> <span class="n">numpy</span><span class="o">.</span><span class="n">timedelta64</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="s1">'us'</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">5</span><span class="p">]:</span> <span class="n">timedelta64</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="s1">'us'</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">6</span><span class="p">]:</span> <span class="nb">print</span> <span class="n">numpy</span><span class="o">.</span><span class="n">timedelta64</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="s1">'us'</span><span class="p">)</span>
<span class="mi">0</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">00.000010</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">7</span><span class="p">]:</span> <span class="nb">print</span> <span class="n">numpy</span><span class="o">.</span><span class="n">timedelta64</span><span class="p">(</span><span class="mf">3600.2</span><span class="p">,</span> <span class="s1">'m'</span><span class="p">)</span> <span class="c1"># decimal part is lost</span>
<span class="mi">2</span> <span class="n">days</span><span class="p">,</span> <span class="mi">12</span><span class="p">:</span><span class="mi">00</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">8</span><span class="p">]:</span> <span class="n">t1</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">zeros</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"datetime64[ms]"</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">9</span><span class="p">]:</span> <span class="n">t2</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"datetime64[ms]"</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">10</span><span class="p">]:</span> <span class="n">t</span> <span class="o">=</span> <span class="n">t2</span> <span class="o">-</span> <span class="n">t1</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">11</span><span class="p">]:</span> <span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="n">datetime</span><span class="o">.</span><span class="n">timedelta</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">24</span><span class="p">)</span> <span class="c1"># setter in action</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">12</span><span class="p">]:</span> <span class="nb">print</span> <span class="n">t</span>
<span class="p">[</span><span class="mi">0</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">24.000</span> <span class="mi">0</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">01.000</span> <span class="mi">0</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">01.000</span> <span class="mi">0</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">01.000</span> <span class="mi">0</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">01.000</span><span class="p">]</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">13</span><span class="p">]:</span> <span class="nb">print</span> <span class="nb">repr</span><span class="p">(</span><span class="n">t</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">13</span><span class="p">]:</span> <span class="n">array</span><span class="p">([</span><span class="mi">24000</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">],</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"timedelta64[ms]"</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">14</span><span class="p">]:</span> <span class="n">t</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">item</span><span class="p">()</span> <span class="c1"># getter in action</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">14</span><span class="p">]:</span> <span class="n">datetime</span><span class="o">.</span><span class="n">timedelta</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">24</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">15</span><span class="p">]:</span> <span class="nb">print</span> <span class="n">t</span><span class="o">.</span><span class="n">dtype</span>
<span class="n">dtype</span><span class="p">(</span><span class="s1">'timedelta64[s]'</span><span class="p">)</span>
</pre></div>
</div>
</section>
<section id="operating-with-date-time-arrays">
<h2>Operating with date/time arrays<a class="headerlink" href="#operating-with-date-time-arrays" title="Link to this heading">#</a></h2>
<section id="datetime64-vs-datetime64">
<h3><code class="docutils literal notranslate"><span class="pre">datetime64</span></code> vs <code class="docutils literal notranslate"><span class="pre">datetime64</span></code><a class="headerlink" href="#datetime64-vs-datetime64" title="Link to this heading">#</a></h3>
<p>The only arithmetic operation allowed between absolute dates is the
subtraction:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">10</span><span class="p">]:</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s2">"M8[s]"</span><span class="p">)</span> <span class="o">-</span> <span class="n">numpy</span><span class="o">.</span><span class="n">zeros</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s2">"M8[s]"</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">10</span><span class="p">]:</span> <span class="n">array</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">],</span> <span class="n">dtype</span><span class="o">=</span><span class="n">timedelta64</span><span class="p">[</span><span class="n">s</span><span class="p">])</span>
</pre></div>
</div>
<p>But not other operations:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">11</span><span class="p">]:</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s2">"M8[s]"</span><span class="p">)</span> <span class="o">+</span> <span class="n">numpy</span><span class="o">.</span><span class="n">zeros</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s2">"M8[s]"</span><span class="p">)</span>
<span class="ne">TypeError</span><span class="p">:</span> <span class="n">unsupported</span> <span class="n">operand</span> <span class="nb">type</span><span class="p">(</span><span class="n">s</span><span class="p">)</span> <span class="k">for</span> <span class="o">+</span><span class="p">:</span> <span class="s1">'numpy.ndarray'</span> <span class="ow">and</span> <span class="s1">'numpy.ndarray'</span>
</pre></div>
</div>
<p>Comparisons between absolute dates are allowed.</p>
<section id="casting-rules">
<h4>Casting rules<a class="headerlink" href="#casting-rules" title="Link to this heading">#</a></h4>
<p>When operating (basically, only the subtraction will be allowed) two
absolute times with different unit times, the outcome would be to raise
an exception. This is because the ranges and time-spans of the different
time units can be very different, and it is not clear at all what time
unit will be preferred for the user. For example, this should be
allowed:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"M8[Y]"</span><span class="p">)</span> <span class="o">-</span> <span class="n">numpy</span><span class="o">.</span><span class="n">zeros</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"M8[Y]"</span><span class="p">)</span>
<span class="go">array([1, 1, 1], dtype="timedelta64[Y]")</span>
</pre></div>
</div>
<p>But the next should not:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"M8[Y]"</span><span class="p">)</span> <span class="o">-</span> <span class="n">numpy</span><span class="o">.</span><span class="n">zeros</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"M8[ns]"</span><span class="p">)</span>
<span class="go">raise numpy.IncompatibleUnitError # what unit to choose?</span>
</pre></div>
</div>
</section>
</section>
<section id="datetime64-vs-timedelta64">
<h3><code class="docutils literal notranslate"><span class="pre">datetime64</span></code> vs <code class="docutils literal notranslate"><span class="pre">timedelta64</span></code><a class="headerlink" href="#datetime64-vs-timedelta64" title="Link to this heading">#</a></h3>
<p>It will be possible to add and subtract relative times from absolute
dates:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">10</span><span class="p">]:</span> <span class="n">numpy</span><span class="o">.</span><span class="n">zeros</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="s2">"M8[Y]"</span><span class="p">)</span> <span class="o">+</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="s2">"m8[Y]"</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">10</span><span class="p">]:</span> <span class="n">array</span><span class="p">([</span><span class="mi">1971</span><span class="p">,</span> <span class="mi">1971</span><span class="p">,</span> <span class="mi">1971</span><span class="p">,</span> <span class="mi">1971</span><span class="p">,</span> <span class="mi">1971</span><span class="p">],</span> <span class="n">dtype</span><span class="o">=</span><span class="n">datetime64</span><span class="p">[</span><span class="n">Y</span><span class="p">])</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">11</span><span class="p">]:</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="s2">"M8[Y]"</span><span class="p">)</span> <span class="o">-</span> <span class="mi">2</span> <span class="o">*</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="s2">"m8[Y]"</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">11</span><span class="p">]:</span> <span class="n">array</span><span class="p">([</span><span class="mi">1969</span><span class="p">,</span> <span class="mi">1969</span><span class="p">,</span> <span class="mi">1969</span><span class="p">,</span> <span class="mi">1969</span><span class="p">,</span> <span class="mi">1969</span><span class="p">],</span> <span class="n">dtype</span><span class="o">=</span><span class="n">datetime64</span><span class="p">[</span><span class="n">Y</span><span class="p">])</span>
</pre></div>
</div>
<p>But not other operations:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">12</span><span class="p">]:</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="s2">"M8[Y]"</span><span class="p">)</span> <span class="o">*</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span> <span class="s2">"m8[Y]"</span><span class="p">)</span>
<span class="ne">TypeError</span><span class="p">:</span> <span class="n">unsupported</span> <span class="n">operand</span> <span class="nb">type</span><span class="p">(</span><span class="n">s</span><span class="p">)</span> <span class="k">for</span> <span class="o">*</span><span class="p">:</span> <span class="s1">'numpy.ndarray'</span> <span class="ow">and</span> <span class="s1">'numpy.ndarray'</span>
</pre></div>
</div>
<section id="id8">
<h4>Casting rules<a class="headerlink" href="#id8" title="Link to this heading">#</a></h4>
<p>In this case the absolute time should have priority for determining the
time unit of the outcome. That would represent what the people wants to
do most of the times. For example, this would allow to do:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">series</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="s1">'1970-01-01'</span><span class="p">,</span> <span class="s1">'1970-02-01'</span><span class="p">,</span> <span class="s1">'1970-09-01'</span><span class="p">],</span>
<span class="go">dtype='datetime64[D]')</span>
<span class="gp">>>> </span><span class="n">series2</span> <span class="o">=</span> <span class="n">series</span> <span class="o">+</span> <span class="n">numpy</span><span class="o">.</span><span class="n">timedelta</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="s1">'Y'</span><span class="p">)</span> <span class="c1"># Add 2 relative years</span>
<span class="gp">>>> </span><span class="n">series2</span>
<span class="go">array(['1972-01-01', '1972-02-01', '1972-09-01'],</span>
<span class="go">dtype='datetime64[D]') # the 'D'ay time unit has been chosen</span>
</pre></div>
</div>
</section>
</section>
<section id="timedelta64-vs-timedelta64">
<h3><code class="docutils literal notranslate"><span class="pre">timedelta64</span></code> vs <code class="docutils literal notranslate"><span class="pre">timedelta64</span></code><a class="headerlink" href="#timedelta64-vs-timedelta64" title="Link to this heading">#</a></h3>
<p>Finally, it will be possible to operate with relative times as if they
were regular int64 dtypes <em>as long as</em> the result can be converted back
into a <code class="docutils literal notranslate"><span class="pre">timedelta64</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">10</span><span class="p">]:</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s1">'m8[us]'</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">10</span><span class="p">]:</span> <span class="n">array</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">],</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"timedelta64[us]"</span><span class="p">)</span>
<span class="n">In</span> <span class="p">[</span><span class="mi">11</span><span class="p">]:</span> <span class="p">(</span><span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s1">'m8[M]'</span><span class="p">)</span> <span class="o">+</span> <span class="mi">2</span><span class="p">)</span> <span class="o">**</span> <span class="mi">3</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">11</span><span class="p">]:</span> <span class="n">array</span><span class="p">([</span><span class="mi">27</span><span class="p">,</span> <span class="mi">27</span><span class="p">,</span> <span class="mi">27</span><span class="p">],</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"timedelta64[M]"</span><span class="p">)</span>
</pre></div>
</div>
<p>But:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>In [12]: numpy.ones(5, 'm8') + 1j
TypeError: the result cannot be converted into a ``timedelta64``
</pre></div>
</div>
<section id="id9">
<h4>Casting rules<a class="headerlink" href="#id9" title="Link to this heading">#</a></h4>
<p>When combining two <code class="docutils literal notranslate"><span class="pre">timedelta64</span></code> dtypes with different time units the
outcome will be the shorter of both (“keep the precision” rule). For
example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">10</span><span class="p">]:</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s1">'m8[s]'</span><span class="p">)</span> <span class="o">+</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s1">'m8[m]'</span><span class="p">)</span>
<span class="n">Out</span><span class="p">[</span><span class="mi">10</span><span class="p">]:</span> <span class="n">array</span><span class="p">([</span><span class="mi">61</span><span class="p">,</span> <span class="mi">61</span><span class="p">,</span> <span class="mi">61</span><span class="p">],</span> <span class="n">dtype</span><span class="o">=</span><span class="s2">"timedelta64[s]"</span><span class="p">)</span>
</pre></div>
</div>
<p>However, due to the impossibility to know the exact duration of a
relative year or a relative month, when these time units appear in one
of the operands, the operation will not be allowed:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">In</span> <span class="p">[</span><span class="mi">11</span><span class="p">]:</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s1">'m8[Y]'</span><span class="p">)</span> <span class="o">+</span> <span class="n">numpy</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s1">'m8[D]'</span><span class="p">)</span>
<span class="k">raise</span> <span class="n">numpy</span><span class="o">.</span><span class="n">IncompatibleUnitError</span> <span class="c1"># how to convert relative years to days?</span>
</pre></div>
</div>
<p>In order to being able to perform the above operation a new NumPy
function, called <code class="docutils literal notranslate"><span class="pre">change_timeunit</span></code> is proposed. Its signature will
be:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">change_timeunit</span><span class="p">(</span><span class="n">time_object</span><span class="p">,</span> <span class="n">new_unit</span><span class="p">,</span> <span class="n">reference</span><span class="p">)</span>
</pre></div>
</div>
<p>where ‘time_object’ is the time object whose unit is to be changed,
‘new_unit’ is the desired new time unit, and ‘reference’ is an absolute