I support Karl's proposal. This is a great improvement, and resolves some serious issues of ambiguity.
However, I suggest expanding the calendar definitions slightly in section 4.1 and minimise the discussion of different calendars in the introduction of chapter 4. In Karl's text there are, I think, too many conditional statements which could cause confusion. Placing the rules for each calendar in the relevant part of section 4.1 could simply the text. In section 4.1: `julian`: -- Julian calendar. Date/times earlier than 1-1-1 0:0:0 are prohibited. Year 1 corresponds to AD 1 in the Julian calendar. `gregorian` or `standard` -- Mixed Gregorian/Julian calendar as defined by Udunits. This is the default. Date/times earlier than 1-1-1 0:0:0 are prohibited. Year 1 corresponds to AD 1 in the Gregorian calendar. `proleptic_gregorian`. -- A Gregorian calendar extended to dates before 1582-10-15. That is, a year is a leap year if either (i) it is divisible by 4 but not by 100 or (ii) it is divisible by 400. Year 1 corresponds to AD 1 and year 0 corresponds to 1BC in the proleptic Gregorian calendar. In the introduction, replace: ``` 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. ``` with: ``` The year number in the reference date/time may be zero or negative, except in the case of a `julian` or `standard`/`gregorian` calendars (see Section 4.1 below for more details). ``` Note: * inclusion of year zero is clearly stated in this first sentence, so it doesn't need to be repeated. * The connection to AD/BC is easier to discuss in the context of the calendars for which it is relevant. * The wikipedia article on ISO 8601 is useful, but using it as a link in the context of the AD/BC question is misleading: ISO 8601 does not mention AD/BC .. it just states that years should be numbered sequentially, with 0 preceding 1. If a reference for AD/BC is needed, there is a good discussion in [Christian chronology](https://urldefense.us/v3/__https://www.britannica.com/topic/anno-Domini-Christian-chronology__;!!G2kpM7uM-TzIFchu!gzPdjGl_BIP7T8Hjpp5NMyDBEmUAeLBgSumAm3WyhP_BgyLZGgs47ypibhOYuvt5tIfykv5FDqY$ ) in the Encyclopedia Britannica and elsewhere, but I'm not sure that we need to discuss it in the Conventions document. The AD/BC system appears to predate the introduction of zero into common usage by the Christian record keepers. Placing references to AD and BC in the relevant calendar definitions makes their meaning clear. -- 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-827429517__;Iw!!G2kpM7uM-TzIFchu!gzPdjGl_BIP7T8Hjpp5NMyDBEmUAeLBgSumAm3WyhP_BgyLZGgs47ypibhOYuvt5tIfyXHAUvxE$ 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].
