Parsing "months since ..." dates

[davidhassell]

Hi,

Following on from #68, it turns out that you can't now parse dates of "months since ..." in the Gregorian calendar:

>>> import cftime
>>> cftime.__version__
'1.6.2'
>>> cftime._dateparse('months since 2000-1-1', 'gregorian')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[17], line 1
----> 1 cftime._dateparse('months since 2000-1-1', 'gregorian')

File src/cftime/_cftime.pyx:98, in cftime._cftime._dateparse()

ValueError: 'months since' units only allowed for '360_day' calendar

Was this intended?

This makes it hard to read datasets like:

	float32 T(T) ;
		T:standard_name = "time" ;
		T:calendar = "standard" ;
		T:units = "months since 1960-01-01" ;

       data:
             T = 0, 1, 2, 3

which exist, even if the creators probably didn't really mean what they encoded (although they might have!).

The lines https://github.com/Unidata/cftime/blob/master/src/cftime/_cftime.pyx#L97-L100 seem to restrictive to me. I'm fine with raising an exception when you try do something in these units (like date2num), but I think that it might be too restrictive to prevent these units from being stored in the first place.

Thanks,
David

[jswhit]

cftime._dateparse is not part of the public api, it's used internally in num2date and date2num. So, unless I'm missing something, you shouldn't ever see this error unless you try to do something with these units.

[jswhit]

Having said that, if you are using cftime._dateparse and don't want it to fail on invalid units I see no harm in removing that error - it will be triggered later in date2num and num2date anyway.

[davidhassell]

Thanks - that would certainly help me! I appreciate that it's an underscore method, but it is very useful - converting CF-netCDF units attributes to datetime objects is something I do frequently, and I don't know of another method for doing it.

[davidhassell]

Ah, but looking at my code, it may be that I don't have to rely on this method as much as I perhaps thought ... I shall investigate :)