Merge 449be55 into 3c18ced

Changelog

@@ -1,6 +1,19 @@
-since version 1.2.1
-===================
+version 1.3.0 (not yet released)
+================================
  * zero pad years in strtime (issue #194)
+ * have `cftime.datetime` create calendar-specific instances that support
+   addition and subtraction (issue #198). Name of base class changed
+   from `cftime.datetime` to `cftime.datetime_base` since `cftime.datetime`
+   is not a factory function.  Iif your are using 
+   `isinstance(my_datetime, cftime.datetime` this will need to be changed
+   to `isinstance(my_datetime, cftime.datetime_base)`. To maintain backward
+   compatibility with older versions of cftime, use
+   ```python
+   try:
+       from cftime import datetime_base
+   except ImportError:
+       datetime_base = datetime
+   ```
 
 version 1.2.1 (release tag v1.2.1rel)
 =====================================

cftime/__init__.py

@@ -1,6 +1,6 @@
 from ._cftime import utime, JulianDayFromDate, DateFromJulianDay, UNIT_CONVERSION_FACTORS
-from ._cftime import _parse_date, date2index, time2index
-from ._cftime import datetime, real_datetime
+from ._cftime import _parse_date, date2index, time2index, DATE_TYPES
+from ._cftime import datetime, real_datetime, datetime_base
 from ._cftime import DatetimeNoLeap, DatetimeAllLeap, Datetime360Day, DatetimeJulian, \
                          DatetimeGregorian, DatetimeProlepticGregorian
 from ._cftime import microsec_units, millisec_units, \