I am in favor of the intent of the changes. With respect to the wording, I offer two general suggestions:
- I think in reference to dates and calendars, we should avoid the terms "real-world", and, rather, say exactly what we mean by it. - I think the references to and description of the possible use of year 0 to represent climatological time are very confusing and need to be rewritten. Specifically, I suggest: Replace the paragraph, ``` The year number in the reference date/time may be zero or negative, except where prohibited for certain calendars, as noted below. A negative year number is indicated with a preceding minus sign. When year zero is allowed, year numbering is algebraic; i.e., year zero is included in the counting. The interpretation for historical year numbering is consistent with link:$$https://en.wikipedia.org/wiki/ISO_8601$$[ISO 8601] (modern revisions); by convention, year zero = 1 BC, year -1 = 2 BC, and so on. ``` with ``` The year number in the reference date/time may be zero or negative, except in the case of a ``julian`` calendar or a mixed Gregorian/Julian calendar (denoted by either ``standard`` or ``gregorian``). When allowed, a negative year number is indicated with a preceding minus sign, and year numbering includes year 0 (in contrast with the Anno Domini (AD) system of numbering years commonly used by western historical scholars in which there is no year 0). The inclusion of year zero in some CF calendars _is_ consistent with astronomical year numbering and with link:$$https://en.wikipedia.org/wiki/ISO_8601$$[ISO 8601] (modern revisions), which stipulate that year zero = 1 BC, year -1 = 2 BC, and so on. ``` Also replace the paragraph, ``` There is a special meaning for year zero in the real-world calendar, that is not recommended for current work. See <<climatological-statistics>>. This is an exception to the prohibition of year zero for certain calendars. ``` with ``` The one exception to the prohibition of year zero for certain calendars is in the now deprecated special use of year zero to indicate a climatology (see <<climatological-statistics>>). ``` In the conformance document, isn't the second sentence of the following wrong? ``` The use of a reference date/time in the year 0 to indicate climatological time is deprecated. This restriction only applies to the real-world calendar as used by the udunits package. ``` This might be interpreted as meaning that use of year 0 for climatology is o.k. for non-real-world calendars, but isn't such use absolutely restricted for any calendar that includes year 0 as a possible real year (like, for example, the proleptic_gregorian calendar)? If so, the above paragraph needs to be reworded. This might not belong in this ticket, but in the climatological-statistics section, I would replace the paragraph ``` The COARDS standard offers limited support for climatological time. For compatibility with COARDS, time coordinates should also be recognised as climatological if they have a units attribute of time-units relative to midnight on 1 January in year 0 i.e. since 0-1-1 in udunits syntax, and provided they refer to the real-world calendar. We do not recommend this convention because (a) it does not provide any information about the intervals used to compute the climatology, and (b) there is no standard for how dates since year 1 will be encoded with units having a reference time in year 0, since this year does not exist; consequently there may be inconsistencies among software packages in the interpretation of the time coordinates. Year 0 may be a valid year in non-real-world calendars, and therefore cannot be used to signal climatological time in such cases. ``` with ``` For compatibility with the COARDS standard, a climatological time coordinate may (for certain calendars) be indicated by setting the time coordinate's units attribute to midnight on 1 January in year 0 (i.e., since 0-1-1), although this is no longer recommended. Use of year 0 for this purpose is absolutely forbidden in the case of a calendar where year 0 is a valid year (i.e., for all calendars except a ``julian`` or a Gregorian/Julian calendar). The reasons for generally avoiding the special use of year 0 to indicate a climatology are: a) it does not provide any information about the intervals used to compute the climatology, and b) there may be inconsistencies among software packages in the interpretation of the time coordinates with a reference time of year 0. ``` -- You are receiving this because you are subscribed to this thread. Reply to this email directly or view it on GitHub: https://urldefense.us/v3/__https://github.com/cf-convention/cf-conventions/issues/298*issuecomment-813046691__;Iw!!G2kpM7uM-TzIFchu!h46iXgWC0ieeygt2LSeBIoapgvHATxrgVV_2ymwYnEuJvON9XwYoe0UwYJxrbx7PGMgWcRMYB5g$ This list forwards relevant notifications from Github. It is distinct from cf-metad...@cgd.ucar.edu, 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 cf-metadata-unsubscribe-requ...@listserv.llnl.gov.
