Dear John Thanks for your useful summary.
> - udunits should no longer be the reference library for calendar > time. a new reference library is needed, which handles non-standard > calendars. If udunits itself were extended to cover non-real-world calendars, wouldn't that be OK? > - udunit date representation ("n timeUnit since ISO_date") must be > retained for backwards compatibility. "month" and "year" timeUnit > should be redefined in CF version 1.x to refer to calendar fields, > not fixed length time durations. For files using versions previous > to CF 1.x, udunit fixed length semantics should be used. Perhaps it would be safer and more explicit to make "month" and "year" *illegal* in CF 1.x. (In practice, we could assist this by providing our own udunits definition file which didn't contain them.) At the same time we could introduce new units calendar_year and calendar_month, which would require special arithmetic by the date-handling software, and are not convertible to other units of time. > - the grammar for udunit date representations should be defined, > so that multiple libraries can implement it It is not perfectly obvious what it should do. n months since 1st of a month makes sense, but what does "1 calendar_month since 31 Jan 2008" mean, for instance. > - ISO date strings should be allowed Does ISO support non-real-world calendars? A problem with allowing strings is that they cannot be coordinate variables in CF, so they cannot be required to be monotonic, since they are auxiliary coordinate variables. We would need special rules if we wanted to enforce that. Would we have to define the rules for putting dates in order or is string-sorting using a default collating sequence a reliable method? To me it seems an unnecessary complication to introduce strings to represent dates. What we need is easy interconversion between strings and numbers, to make the files human-readable. ncdump supports this now, doesn't it, and is aware of CF calendars. Isn't that good enough? Best wishes Jonathan _______________________________________________ CF-metadata mailing list CF-metadata@cgd.ucar.edu http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata