'standard' calendar refers to 'proleptic_gregorian' in cftime_range rather than 'gregorian'

[spencerkclark]

Code Sample, a copy-pastable example if possible

In [1]: import xarray

In [2]: xarray.cftime_range('2000', periods=3, calendar='standard').values
Out[2]:
array([cftime.DatetimeProlepticGregorian(2000, 1, 1, 0, 0, 0, 0, -1, 1),
       cftime.DatetimeProlepticGregorian(2000, 1, 2, 0, 0, 0, 0, -1, 1),
       cftime.DatetimeProlepticGregorian(2000, 1, 3, 0, 0, 0, 0, -1, 1)],
      dtype=object)

Problem description

When writing cftime_range I used dates from a proleptic Gregorian calendar when the calendar type was specified as 'standard'. While this is consistent with Python's built-in datetime.datetime (which uses a proleptic Gregorian calendar), this differs from the behavior in cftime.num2date and ultimately the CF conventions, which state that 'standard' should refer to the true Gregorian calendar. My inclination is that considering "cf" is in the name of cftime_range, we should adhere to those conventions as closely as possible (and hence the way I initially coded things was a mistake).

Expected Output

In [2]: xarray.cftime_range('2000', periods=3, calendar='standard').values
Out[2]:
array([cftime.DatetimeGregorian(2000, 1, 1, 0, 0, 0, 0, -1, 1),
       cftime.DatetimeGregorian(2000, 1, 2, 0, 0, 0, 0, -1, 1),
       cftime.DatetimeGregorian(2000, 1, 3, 0, 0, 0, 0, -1, 1)],
      dtype=object)

Do others agree that we should fix this? If we were to make this change, would it be appropriate to consider it a bug and simply make the breaking change immediately, or might we need a deprecation cycle?

[shoyer]

Yes, I think we can consider this a bug and just fix it.

[spencerkclark]

Sounds good; see #2771.