Can you point me to where in the CF Conventions or UDUNITS the valid time coordinate units defined?

This edge case is defined here: http://cfconventions.org/Data/cf-conventions/cf-conventions-1.6/build/cf-conventions.html#climatological-statistics

NB: People that follow the COARDS conventions will also need this. Any Udunits wrapper can deal with that for you:

``` python import cf_units u = cf_units.Unit('days since 0000-01-01 00:00:00', calendar=cf_units.CALENDAR_NO_LEAP) ut = u.utime()

Returns a fake datetime object (See http://scitools.org.uk/iris/docs/latest/iris/iris/unit.html#iris.unit.Unit.num2date)

ut.num2date(0) 0-01-01 00:00:00

Note that python datetime cannot take year = 0, but udunits did the "right" thing.

ut.date2num(datetime(1, 1, 1)) 365.0 ```

We should think about whether or not this fix belongs in xray or netCDF4. I am of the opinion that if the CF Conventions do in fact support the units in question, we should apply this fix in the netCDF package. If they don't, I don't think we wan to support it here either.

I strongly disagree with that. netCDF is not one CDM and should not follow the CF-conventions! I know that iris does follow CF very closely (annoyingly in fact) and xray "kind of" follow (which is great BTW). However, if such conventions were adopted in the netCDF package, how will we load non-CF files?