add 'months since' for 360_day calendar (issue #68)

cftime/_cftime.pyx

@@ -22,6 +22,7 @@ sec_units = ['second', 'seconds', 'sec', 'secs', 's']
 min_units = ['minute', 'minutes', 'min', 'mins']
 hr_units = ['hour', 'hours', 'hr', 'hrs', 'h']
 day_units = ['day', 'days', 'd']
+month_units = ['month', 'months']
 _units = microsec_units+millisec_units+sec_units+min_units+hr_units+day_units
 # supported calendars. Includes synonyms ('standard'=='gregorian',
 # '366_day'=='all_leap','365_day'=='noleap')
@@ -104,12 +105,15 @@ def _round_half_up(x):
  # 'round half up' so 0.5 rounded to 1 (instead of 0 as in numpy.round)
  return np.ceil(np.floor(2.*x)/2.)
 
-cdef _parse_date_and_units(timestr):
+cdef _parse_date_and_units(timestr,calendar='standard'):
  """parse a string of the form time-units since yyyy-mm-dd hh:mm:ss
  return a tuple (units,utc_offset, datetimeinstance)"""
  (units, isostring) = _datesplit(timestr)
- if units not in _units:
- raise ValueError(
+ if not ((units in month_units and calendar=='360_day') or units in _units):
+ if units in month_units and calendar != '360_day':
+ raise ValueError("'months since' units only allowed for '360_day' calendar")
+ else:
+ raise ValueError(
  "units must be one of 'seconds', 'minutes', 'hours' or 'days' (or singular version of these), got '%s'" % units)
  # parse the date string.
  year, month, day, hour, minute, second, utc_offset = _parse_date(
@@ -710,7 +714,8 @@ units to datetime objects.
  else:
  raise ValueError(
  "calendar must be one of %s, got '%s'" % (str(_calendars), calendar))
- units, tzoffset, self.origin = _parse_date_and_units(unit_string)
+ units, tzoffset, self.origin =\
+ _parse_date_and_units(unit_string,calendar)
  # real-world calendars limited to positive reference years.
  if self.calendar in ['julian', 'standard', 'gregorian', 'proleptic_gregorian']:
  if self.origin.year == 0:
@@ -774,6 +779,8 @@ units to datetime objects.
  jdelta = jdelta * 24. + self.tzoffset / 60.
  elif self.units in day_units:
  jdelta = jdelta + self.tzoffset / 1440.
+ elif self.units in month_units and self.calendar == '360_day':
+ jdelta = jdelta/30. + self.tzoffset / (30. * 1440.)
  else:
  raise ValueError('unsupported time units')
  if isscalar:
@@ -828,6 +835,9 @@ units to datetime objects.
  jdelta = time_value / 24. - self.tzoffset / 1440.
  elif self.units in day_units:
  jdelta = time_value - self.tzoffset / 1440.
+ elif self.units in month_units and self.calendar == '360_day':
+ # only allowed for 360_day calendar
+ jdelta = time_value * 30. - self.tzoffset / 1440.
  else:
  raise ValueError('unsupported time units')
  jd = self._jd0 + jdelta

test/test_cftime.py

@@ -654,7 +654,11 @@ def runTest(self):
  t = date2num(datetime(1985,1,2), units, calendar="standard")
  assert_almost_equal(t, 2446068)
 
-
+ # issue #68: allow months since for 360_day calendar
+ d = num2date(1, 'months since 0000-01-01 00:00:00', calendar='360_day')
+ self.assertEqual(d, Datetime360Day(0,2,1))
+ t = date2num(d, 'months since 0000-01-01 00:00:00', calendar='360_day')
+ self.assertEqual(t, 1)
 
 
 class TestDate2index(unittest.TestCase):