Merge pull request #178 from spencerkclark/exact-date2num

Make cftime.datetime - cftime.datetime -> timedelta microsecond-exact

cftime/_cftime.pyx

@@ -1793,8 +1793,14 @@ Gregorial calendar.
                     raise ValueError("cannot compute the time difference between dates with different calendars")
                 if dt.calendar == "":
                     raise ValueError("cannot compute the time difference between dates that are not calendar-aware")
-                converter = _converters[dt.calendar]
-                return timedelta(seconds=converter.date2num(dt) - converter.date2num(other))
+                ordinal_self = _IntJulianDayFromDate(dt.year, dt.month, dt.day, dt.calendar)
+                ordinal_other = _IntJulianDayFromDate(other.year, other.month, other.day, other.calendar)
+                days = ordinal_self - ordinal_other
+                seconds_self = dt.second + 60 * dt.minute + 3600 * dt.hour
+                seconds_other = other.second + 60 * other.minute + 3600 * other.hour
+                seconds = seconds_self - seconds_other
+                microseconds = dt.microsecond - other.microsecond
+                return timedelta(days, seconds, microseconds)
             elif isinstance(other, datetime_python):
                 # datetime - real_datetime
                 if not dt.datetime_compatible:

test/test_cftime.py

@@ -1583,6 +1583,14 @@ def test_dayofyr_after_timedelta_addition(date_type):
     assert date_after_timedelta_addition.dayofyr == 3
 
 
+def test_exact_datetime_difference(date_type):
+    b = date_type(2000, 1, 2, 0, 0, 0, 5)
+    a = date_type(2000, 1, 2)
+    result = b - a
+    expected = timedelta(microseconds=5)
+    assert result == expected
+
+
 _SHAPES = [(4, ), (2, 2)]
 _MICROSECOND_UNITS = ["microseconds", "microsecond", "microsec", "microsecs"]
 _MILLISECOND_UNITS = ["milliseconds", "millisecond", "millisec", "millisecs", "ms"]