Well I see that my feeble attempts to avoid the year zero issue have failed. I think that the focus in this conversation is good, and I hope we can reach a clear resolution for [all six CF defined calendars](http://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#calendar). Thanks everyone so far, for your research and attention to detail.
I support the ISO 8601 "expanded representation" approach for the interpretation of year numbering. ISO 8601 deals not specifically with calendar definitions, but rather with how to construct string representations. This is relevant for the reference time in the CF units string. As reported by @peterkuma, this representation puts year numbers on a mathematically normal integer time axis that includes negative years and year zero. @martinjuckes, I would prefer to stay with the familiar year-month-day string syntax, and avoid the new explicit ISO 8601 form that adds new designators such as "Y" and "YB". I think it will be sufficient to simply add explicit documentation for the proper CF treatment of negative and zero year numbers in the units string. I favor adding constraints for two of the CF calendars. This is a different way of avoiding the year zero issue, specifically an attempt to keep most common applications "safe" from crossover problems. The calendar named "Gregorian" should be restricted to only dates from 1582 October 15 forward. This would apply to both the reference date and all encoded dates. This should be fully compatible with existing data sets that have paid any attention to stated best practices for many years now. Likewise, the calendar named "Julian" should be restricted to only 0001 January 1 forward. As a result, the need to clarify negative and zero years would be reduced to only the remaining four [CF calendars](http://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#calendar): 360_day, 365_day, 366_day, and proleptic_gregorian. -- You are receiving this because you are subscribed to this thread. Reply to this email directly or view it on GitHub: https://github.com/cf-convention/cf-conventions/issues/298#issuecomment-698504236 This list forwards relevant notifications from Github. It is distinct from [email protected], although if you do nothing, a subscription to the UCAR list will result in a subscription to this list. To unsubscribe from this list only, send a message to [email protected].
