Bump version v9.2.1 - documentation change only

doc(Period,Duration): improve
- Explain Duration and Period better as to there difference. Fix an error in the doc for Duration as well.
- Add warning in README.md to indicate that Duration and Period are reversed in date-elm-extra in meaning compared to nodatime, this is purely due to me mixing them up with out realising it, I have not yet figured out if I should swap the concepts around in elm-date-extra or not at the moment.

README.md

@@ -1,9 +1,17 @@
 # Date Extra Package
 
+
+# WARNING!
+
+I have discovered that I managed to copy concepts from NodaTimes Period concept to Duration and there Duration concept to Period I got it backwards. Which I am sure is not helping anyone with back ground in nodatime doing the right thing with elm-date-extra.
+
+* I am not sure if i should swap them in elm-date-extra so they match the NodaTime terms at the moment.
+
 ### Introduction
 
 An Elm language package for working with dates and times.
 
+
 See bottom of this document for important history notes.
 
 ### Code format

elm-package.json

@@ -1,5 +1,5 @@
 {
-    "version": "9.2.0",
+    "version": "9.2.1",
     "summary": "Date Extra library add/subtract/diff/format etc dates.",
     "repository": "https://github.com/rluiten/elm-date-extra.git",
     "license": "BSD3",

src/Date/Extra/Duration.elm

@@ -11,6 +11,10 @@ module Date.Extra.Duration
 {-| A Duration is a length of time that may vary with calendar date
 and time. It can be used to modify a date.
 
+Represents a period of time expressed in human chronological terms
+in terms of a calendar which may have varying components based upon
+the dates involved in the math.
+
 When modify dates using Durations (Day | Month | Week | Year) this module
 compensates for day light saving hour variations to minimise the scenarios
 that cause the Hour field in the result to be different to the input date.
@@ -303,7 +307,10 @@ addYear yearCount date =
 {-| Return a Period representing date difference. date1 - date2.
 
 If you add the result of this function to date2 with addend of 1
-will return date1.
+will not always return date1, this is because this module supports
+human calendar concepts liek Day Light Saving, Months with varying
+number of days dependent on the month and leap years. So the difference
+between two dates is dependent on when those dates are.
 
 **Differences to Period.diff**
 

src/Date/Extra/Period.elm

@@ -1,16 +1,18 @@
 module Date.Extra.Period
     exposing
-        ( add
-        , DeltaRecord
-        , diff
+        ( DeltaRecord
         , Period(..)
+        , add
+        , diff
         , toTicks
         , zeroDelta
         )
 
 {-| Period is a fixed length of time. It is an elapsed time concept, which
 does not include the concept of Years Months or Daylight saving variations.
 
+  - Represents a fixed (and calendar-independent) length of time.
+
 Name of type concept copied from NodaTime.
 
 @docs add
@@ -162,18 +164,18 @@ diff date1 date2 =
                     absDayDiff =
                         abs onlyDaysDiff
                 in
-                    ( negate (absDayDiff // 7)
-                    , negate (absDayDiff % 7)
-                    )
+                ( negate (absDayDiff // 7)
+                , negate (absDayDiff % 7)
+                )
             else
                 ( onlyDaysDiff // 7
                 , onlyDaysDiff % 7
                 )
     in
-        { week = weekDiff
-        , day = dayDiff
-        , hour = hourDiff
-        , minute = minuteDiff
-        , second = secondDiff
-        , millisecond = millisecondDiff
-        }
+    { week = weekDiff
+    , day = dayDiff
+    , hour = hourDiff
+    , minute = minuteDiff
+    , second = secondDiff
+    , millisecond = millisecondDiff
+    }