Improved macros for determing MJD from year/month. This extends the r…

…ange over which the macros can be used. Also, there were slight differences in the definitions and usable time spans in the three files where they're used.
Bill-Gray · Apr 15, 2024 · b71dbc1 · b71dbc1
1 parent 38c6afa
commit b71dbc1
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Showing 3 changed files with 40 additions and 30 deletions.
diff --git a/delta_t.cpp b/delta_t.cpp
@@ -361,14 +361,17 @@ long double DLL_FUNC tdb_minus_tdt( const long double t_centuries)
    return( rval);     /* difference is in _seconds_  */
 }
 
-/* These macros determine the MJD of the given date in 'YEAR'.  They're
-valid for years after -9999 in the _Gregorian_ calendar.  Note that
-we use the fact that February 1 is always 31 days after January 1,
-but work backward from January 1 of the following year to get
-March to December without having to consider leap days. */
-
-#define JAN_1( YEAR) (((YEAR) * 365 + ((YEAR) + 9999) / 4 - ((YEAR) + 9999) / 100 \
-                         + ((YEAR) + 9999) / 400) - 681365)
+/* These macros determine the MJD of the given date in 'YEAR'. They
+are valid for (Gregorian) years after roughly 20 billion years ago.
+This was chosen to comfortably include all dates since the Big Bang.
+We use the fact that February 1 is always 31 days after January 1,
+but have to work backward from January 1 of the following year to
+get March to December without having to consider leap days. */
+
+#define BASE_YEAR 19999999999L
+#define JAN_1( YEAR) (((YEAR) * 365L + ((YEAR) + BASE_YEAR) / 4L - ((YEAR) + BASE_YEAR) / 100L \
+                         + ((YEAR) + BASE_YEAR) / 400L) - 678940L \
+                         - ((BASE_YEAR + 1L) / 400L) * 97L)
 #define FEB_1( YEAR) (JAN_1( YEAR) + 31)
 #define DEC_1( YEAR) (JAN_1( (YEAR)+1) - 31)
 #define NOV_1( YEAR) (DEC_1( YEAR) - 30)

diff --git a/utc_algo.cpp b/utc_algo.cpp
@@ -57,10 +57,14 @@ up with a UTC offset that'll keep |UTC-UT| < .5 second;  i.e.,  keep UTC
 and UT as close together as possible.
 
    This is admittedly something of a kludge.  But anything done to estimate
-future leap seconds has to be a kludge;  there's no "good" way to do it.  */
+future leap seconds has to be a kludge;  there's no "good" way to do it.
 
-#define JAN_1( YEAR) (((YEAR) * 365 + ((YEAR) - 1) / 4 - ((YEAR) - 1) / 100 \
-                         + ((YEAR) - 1) / 400) - 678940)
+   See 'delta_t.cpp' for an explanation of the following macro. */
+
+#define BASE_YEAR 19999999999L
+#define JAN_1( YEAR) (((YEAR) * 365L + ((YEAR) + BASE_YEAR) / 4L - ((YEAR) + BASE_YEAR) / 100L \
+                         + ((YEAR) + BASE_YEAR) / 400L) - 678940L \
+                         - ((BASE_YEAR + 1L) / 400L) * 97L)
 
 // unsigned day = mjd + 1931367u + 2400000u;
 // int year = (int)( day * 400u / 146097u) - 10000;
@@ -77,12 +81,12 @@ int main( const int argc, const char **argv)
 
       printf( "Year = %d\n", year);
       low = JAN_1( year);
-      printf( "MJD 1 Jan %d = %d\n", year, JAN_1( year));
+      printf( "MJD 1 Jan %d = %ld\n", year, JAN_1( year));
       if( mjd < low)
          {
          year--;
          low = JAN_1( year);
-         printf( "MJD 1 Jan %d = %d\n", year, JAN_1( year));
+         printf( "MJD 1 Jan %d = %ld\n", year, JAN_1( year));
          }
       high = JAN_1( year + 1);
       printf( "MJD 1 Jan %d = %d\n", year + 1, high);
@@ -109,7 +113,7 @@ int main( const int argc, const char **argv)
             n_less++;
          mjd++;
          }
-      printf( "%d %d\n", n_more, n_less);
+      printf( "%d cases went over;  %d went under\n", n_more, n_less);
       }
    return( 0);
 }
diff --git a/utc_test.cpp b/utc_test.cpp
@@ -15,24 +15,27 @@ along with this program; if not, write to the Free Software
 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 02110-1301, USA. */
 
-/* These macros determine the MJD of the given date in 'YEAR'.         */
-/* They're valid for _non-negative_ years in the _Gregorian_ calendar. */
-/* Some are not used here,  and therefore commented out,  just to      */
-/* avoid compiler warnings.                                            */
+/* These macros determine the MJD of the given date in 'YEAR'.
+They are explained a bit in comments in 'delta_t.cpp' (q.v.).
+We're only using the July and January ones (the months in which
+leap seconds occur).  Unused macros are commented out,  just to
+avoid nuisance compiler warnings.   */
 
-#define JAN_1( YEAR) (((YEAR) * 365 + ((YEAR) - 1) / 4 - ((YEAR) - 1) / 100 \
-                         + ((YEAR) - 1) / 400) - 678940)
+#define BASE_YEAR 19999999999L
+#define JAN_1( YEAR) (((YEAR) * 365L + ((YEAR) + BASE_YEAR) / 4L - ((YEAR) + BASE_YEAR) / 100L \
+                         + ((YEAR) + BASE_YEAR) / 400L) - 678940L \
+                         - ((BASE_YEAR + 1L) / 400L) * 97L)
 // #define FEB_1( YEAR) (JAN_1( YEAR) + 31)
-#define MAR_1( YEAR) (((YEAR)*365 + (YEAR)/4 - (YEAR)/100 + (YEAR)/400) - 678881)
-#define APR_1( YEAR) (MAR_1( YEAR) + 31)
-#define MAY_1( YEAR) (APR_1( YEAR) + 30)
-#define JUN_1( YEAR) (MAY_1( YEAR) + 31)
-#define JUL_1( YEAR) (JUN_1( YEAR) + 30)
-// #define AUG_1( YEAR) (JUL_1( YEAR) + 31)
-// #define SEP_1( YEAR) (AUG_1( YEAR) + 31)
-// #define OCT_1( YEAR) (SEP_1( YEAR) + 30)
-// #define NOV_1( YEAR) (OCT_1( YEAR) + 31)
-// #define DEC_1( YEAR) (NOV_1( YEAR) + 30)
+#define DEC_1( YEAR) (JAN_1( (YEAR)+1) - 31)
+#define NOV_1( YEAR) (DEC_1( YEAR) - 30)
+#define OCT_1( YEAR) (NOV_1( YEAR) - 31)
+#define SEP_1( YEAR) (OCT_1( YEAR) - 30)
+#define AUG_1( YEAR) (SEP_1( YEAR) - 31)
+#define JUL_1( YEAR) (AUG_1( YEAR) - 31)
+// #define JUN_1( YEAR) (JUL_1( YEAR) - 30)
+// #define MAY_1( YEAR) (JUN_1( YEAR) - 31)
+// #define APR_1( YEAR) (MAY_1( YEAR) - 30)
+// #define MAR_1( YEAR) (APR_1( YEAR) - 31)
 
 #include <stdio.h>
 #include <stdlib.h>