decode 'months since' for 360_day calendars

[rabernat]

The IRI Data Library contains tons of datasets with the following time attributes:

	float32 T(T) ;
		T:standard_name = time ;
		T:pointwidth = 1.0 ;
		T:long_name = Time ;
		T:calendar = 360 ;
		T:expires = 1538524800 ;
		T:gridtype = 0 ;
		T:units = months since 1960-01-01 ;

(edited to correct typo in original post)

(example dataset)

I would like to be able to open and decode these datasets in xarray, with time decoding handled by cftime.

There are two problems:

1. The calendar is not a valid CF calendar. It should be 360_day. But that is easy to fix by rewriting the calendar attribute.
2. Even with calendar = 360_day, months is not considered a valid time unit

However, in a 360-day calendar month==30 days, so this should be valid.

This was discussed over in Unidata/netcdf4-python#434 (comment), where @jswhit commented:

A pull request allowing months since when calendar is 360_day would be welcome

[rabernat]

To clarify, currently in cftime, I get the following:

cftime.num2date(0, 'months since 1960-01-01', '360_day')

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-65-4a67652d64b1> in <module>()
----> 1 cftime.num2date(0, 'months since 1960-01-01', '360_day')

cftime/_cftime.pyx in cftime._cftime.num2date()

cftime/_cftime.pyx in cftime._cftime.utime.__init__()

cftime/_cftime.pyx in cftime._cftime._parse_date_and_units()

ValueError: units must be one of 'seconds', 'minutes', 'hours' or 'days' (or singular version of these), got 'months'

[jswhit]

Created a new pull request #75 which includes both the date2num and num2date implementations, and works with the latest updates to master.

[jswhit]

merged pull request #75

[davidhassell]

Sorry, I've not been able to look at this for a few days - does this mean that "months since ..." are always decoded as 30 day months when the calendar is "360_day"?