Time units "months" are not handled correctly

[adrfantini]

Time units "months" are not handled correctly. This is because a month is not a well-defined unit, in fact it is not even CF-compliant. Udunits understands this unit but kinda unofficially (it is not in the official derived units and accepted units list).

Ideally the only practical way to fix this would be to use udunits for time handling, via the units package. This is easy to implement, even possible via an option. I did not initially do this and I do not recall why, maybe to reduce the number of dependencies.