The cf-conventions folks seem to have converged around the following solution for negative years (and year zero).

• For the current standard and julian calendars (and synonyms), zero and negative years are prohibited (cftime disallows year zero but allows negative years back to -4713 in the units string)

• For the current 360_day, 365_day, 366-day, and proleptic_gregorian calendars (and synonyms), year numbering is the complete set of integers, including zero and negative (this is consistent with the current cftime behavior, except for the proleptic_gregorian for which year zero is not allowed and the earliest allowed date is Nov 24 year -4714).

Based on this, the behavior of the proleptic gregorian calendar should be changed in cftime to allow year zero. The Julian and mixed calendars would have to be changed to disallow negative years in the units definition. These changes could be problematic, as many users could be relying on the current behavior.

For the CF discussion, see cf-convention/cf-conventions#298