Update to include MpasRelativeDelta class

Author: xylar

design_docs/timekeeping_reorg.md

@@ -104,24 +104,46 @@ As long as `relativedelta` objects rather than `datetime.timedelta` objects are
 or the 365-day calendar.
 
 <h2> Implementation: Capability of incrementing dates by a number of years and/or months <br>
-Date last modified: 2017/02/06 <br>
+Date last modified: 2017/02/09 <br>
 Contributors: Xylar Asay-Davis
 </h2>
 
 The implementation is on the branch:
 https://github.com/xylar/MPAS-Analysis/tree/timekeeping_reorg
 and in PR #102
 
-The proposed implementation uses `dateutil.relativedelta.relativedelta` to compute the expected 
+The proposed implementation adds a new class MpasRelativeDelta derived from 
+`dateutil.relativedelta.relativedelta` to compute the expected 
 increments in years and months (as well as days, hours, minutes and seconds, as needed).
-The function for converting a date string to a `relativedelta` is documented as follows:
+The class is documented as follows
+```python
+class MpasRelativeDelta(relativedelta):
+    """
+    MpasRelativeDelta is a subclass of dateutil.relativedelta for relative time
+    intervals with different MPAS calendars.
+
+    Only relative intervals (years, months, etc.) are supported and not the
+    absolute date specifications (year, month, etc.).  Addition/subtraction
+    of datetime.datetime objects (but not other MpasRelativeDelta,
+    datetime.timedelta or other related objects) is supported.
+
+    Author
+    ------
+    Xylar Asay-Davis
+
+    Last Modified
+    -------------
+    02/09/2017
+```
+
+The function for converting a date string to a `MpasRelativeDelta` is documented as follows:
 ```python
 from dateutil.relativedelta import relativedelta
 ...
 def stringToRelativedelta(dateString, calendar='gregorian'):
     """
     Given a date string and a calendar, returns an instance of
-    `dateutil.relativedelta.relativedelta`
+    `MpasRelativeDelta`
 
     Parameters
     ----------
@@ -147,7 +169,7 @@ def stringToRelativedelta(dateString, calendar='gregorian'):
 
     Returns
     -------
-    relativedelta : A `dateutil.relativedelta.relativedelta` object
+    relativedelta : An `MpasRelativeDelta` object
 
     Raises
     ------
@@ -174,7 +196,9 @@ Analysis will be run on Edison with all available configurations found in `confi
 are currently no plans to run with the `gregorian` calendar option, we do not have test runs that use this
 calendar.  If this situation changes in the future, we'll test at that time.
 
-Regression tests previously for `Date` has been modified to test the new utility functions.
+Regression tests previously for `Date` has been modified to test the new utility functions. New tests
+have been added to test that dates with both `gregorian` and `gregorian_noleap` calendars behave as
+expected, particularly around the leap day.
 
 <h1> Testing </h1>
 <h2> Testing and Validation: Capability of incrementing dates by a number of years and/or months <br>