Issue SciTools#185: Convert calendars if possible wrt Julian-Gregoria…

…n gap

cf_units/__init__.py

@@ -135,6 +135,10 @@
  CALENDAR_ALL_LEAP: CALENDAR_366_DAY,
 }
 
+#: Calendars that possibly could be converted into each other
+#: depending on min/max date
+GREGORIAN_CALENDARS = {CALENDAR_STANDARD, CALENDAR_PROLEPTIC_GREGORIAN}
+JULIAN_CALENDARS = {CALENDAR_STANDARD, CALENDAR_JULIAN}
 
 #
 # floating point types
@@ -1791,22 +1795,58 @@ def convert(self, value, other, ctype=FLOAT64, inplace=False):
 
  .. note::
 
- Conversion between unit calendars is not permitted unless the
- calendars are aliases, see :attr:`cf_units.CALENDAR_ALIASES`.
-
+ Conversion between unit calendars is permitted only if one of the
+ following conditions are met:
+ * the calendars are aliases, see :attr:`cf_units.CALENDAR_ALIASES`.
+ * the calendars are ``standard`` and ``proleptic_gregorian`` and
+ all `value``s represent dates onwards from 1582-10-15 0:0:0
+ * the calendars are ``standard`` and ``julian`` and all
+ ``value``s represent dates before 1582-10-5 0:0:0
  >>> from cf_units import Unit
- >>> a = Unit('days since 1850-1-1', calendar='gregorian')
+ >>> a = Unit('days since 1850-1-1', calendar='proleptic_gregorian')
  >>> b = Unit('days since 1851-1-1', calendar='standard')
  >>> a.convert(365.75, b)
  0.75
+ >>> a = Unit('days since 1582-10-4', calendar='julian')
+ >>> b = Unit('days since 1582-9-4', calendar='standard')
+ >>> a.convert(0, b)
+ 30.0
+ >>> a = Unit('days since 1583-10-15', calendar='standard')
+ >>> b = Unit('days since 1582-10-15', calendar='proleptic_gregorian')
+ >>> a.convert(0, b)
+ 365
 
  """
+
  other = as_unit(other)
 
  if self == other:
  return value
 
- if self.is_convertible(other):
+ if self.is_time_reference and other.is_time_reference:
+ min_date, max_date = tuple(
+ self.num2date([np.min(value), np.max(value)])
+ )
+ is_gregorian_ok = (
+ {self.calendar, other.calendar} == GREGORIAN_CALENDARS
+ ) and Unit("days since 1582-10-15 ", calendar="standard").date2num(
+ min_date
+ ) >= 0
+ is_julian_ok = (
+ {self.calendar, other.calendar} == JULIAN_CALENDARS
+ ) and Unit("days since 1582-10-5", calendar="julian").date2num(
+ max_date
+ ) < 0
+ else:
+ is_gregorian_ok = False
+ is_julian_ok = False
+
+ if is_gregorian_ok or is_julian_ok:
+ self_newcalendar = Unit(str(self), other.calendar)
+ result = self_newcalendar.convert(
+ value, other, ctype=ctype, inplace=inplace
+ )
+ elif self.is_convertible(other):
  if inplace:
  result = value
  else:
@@ -1888,11 +1928,11 @@ def convert(self, value, other, ctype=FLOAT64, inplace=False):
  # Utilise global convenience dictionary
  # _cv_convert_scalar
  result = _cv_convert_scalar[ctype](ut_converter, result)
- return result
  else:
  raise ValueError(
  "Unable to convert from '%r' to '%r'." % (self, other)
  )
+ return result
 
  @property
  def cftime_unit(self):