handling of year 0 changed in python cftime

[Alexander-Barth]

Describe the bug

The handling of year 0 changed in python cftime:

10/1/2021: Version 1.5.1 released. Changed default behavior of proleptic_gregorian to has_year_zero=T 
(since it is allowed in ISO-8601 and CF does not specify the year zero convention for this calendar). 
Raise warning message when trying to create a calendar that is not supported by CF version 1.9 
(no years < 1 allowed for 'standard'/'gregorian' or 'julian' calendars). 

To Reproduce

using CFTime, Dates
DateTimeProlepticGregorian(1,1,1) - Day(1)
# output: DateTimeProlepticGregorian(-001-12-31T00:00:00)

All calendars are affected expect DateTimeStandard and DateTimeJulian

Expected behavior

output should be DateTimeProlepticGregorian(0,12,31) as in python:

In [3]: cftime.DatetimeProlepticGregorian(1, 1, 1) + timedelta(days=-1)
Out[3]: cftime.DatetimeProlepticGregorian(0, 12, 31, 0, 0, 0, 0, has_year_zero=True)

Environment

• operating system: Ubuntu 20.04
• Julia version: julia 1.7.2, official binaries from https://julialang.org/downloads/

Full output

No additional output. All reference values from python and cftime 1.6.0:

In [3]: cftime.DatetimeProlepticGregorian(1, 1, 1) + timedelta(days=-1)
Out[3]: cftime.DatetimeProlepticGregorian(0, 12, 31, 0, 0, 0, 0, has_year_zero=True)

In [4]: cftime.DatetimeJulian(1, 1, 1) + timedelta(days=-1)
<ipython-input-4-e85e1427bb46>:1: CFWarning: this date/calendar/year zero convention is not supported by CF
  cftime.DatetimeJulian(1, 1, 1) + timedelta(days=-1)
Out[4]: cftime.DatetimeJulian(-1, 12, 31, 0, 0, 0, 0, has_year_zero=False)

In [5]: cftime.DatetimeGregorian(1, 1, 1) + timedelta(days=-1)
<ipython-input-5-d72578c40af5>:1: CFWarning: this date/calendar/year zero convention is not supported by CF
  cftime.DatetimeGregorian(1, 1, 1) + timedelta(days=-1)
Out[5]: cftime.DatetimeGregorian(-1, 12, 31, 0, 0, 0, 0, has_year_zero=False)

In [6]: cftime.DatetimeAllLeap(1, 1, 1) + timedelta(days=-1)
Out[6]: cftime.DatetimeAllLeap(0, 12, 31, 0, 0, 0, 0, has_year_zero=True)

In [7]: cftime.DatetimeNoLeap(1, 1, 1) + timedelta(days=-1)
Out[7]: cftime.DatetimeNoLeap(0, 12, 31, 0, 0, 0, 0, has_year_zero=True)

In [8]: cftime.Datetime360Day(1, 1, 1) + timedelta(days=-1)
Out[8]: cftime.Datetime360Day(0, 12, 30, 0, 0, 0, 0, has_year_zero=True)

[Alexander-Barth]

https://web.archive.org/web/20240304171516/https://unidata.github.io/cftime/api.html

has_year_zero: if set to True, astronomical year numbering is used and the year zero exists. If set to False for real-world calendars, then historical year numbering is used and the year 1 is preceded by year -1 and no year zero exists. The defaults are set to conform with CF version 1.9 conventions (False for ‘julian’, ‘gregorian’/’standard’, True for ‘proleptic_gregorian’ (ISO 8601) and True for the idealized calendars ‘noleap’/’365_day’, ‘360_day’, 366_day’/’all_leap’) The defaults can only be over-ridden for the real-world calendars, for the the idealized calendars the year zero always exists and the has_year_zero kwarg is ignored. This kwarg is not needed to define calendar systems allowed by CF (the calendar-specific defaults do this).

[Alexander-Barth]

The default python-cftime is now implemented:

CFTime.jl/src/query.jl

Lines 5 to 6 in 34630f3

	@inline _hasyear0(::Type{T}) where T <: Union{DateTimeJulian,DateTimeStandard} = false
	@inline _hasyear0(::Type{T}) where T <: AbstractCFDateTime = true