Julian/Gregorian calendar name and constraints
|Reported by:||Dave.Allured||Owned by:||cf-conventions@…|
|Keywords:||calendar time julian gregorian udunits||Cc:|
Julian/Gregorian? calendar name and constraints
An accurate name for the mixed Julian/Gregorian? calendar system is needed as an alternative to the current naming scheme, which hides the complexity and pitfalls of this calendar. Time range constraints enable error checking.
4. Initial Statement of Technical Proposal
However, this is proposed as an independent calendar definition, because it is also desirable to add time range constraints which are not part of the original definitions. Negative years, year zero, and illegal transition dates are all excluded. I am excluding Julian negative years because there is no unambiguous definition in the broader context of general history.
This new definition is fully compatible with existing translation routines in UDUNITS and other calendar software, provided that any name checking procedures are updated. Constraint checking is optional.
This proposal does not change existing usage or create incompatibility with existing data files.
- Provide an accurate and distinct alternate name for this calendar system, for those who wish to be more specific than the default or "standard".
- Facilitate migration away from inadequate calendar names, for both existing and new data sets.
- Increase user awareness of complicated calendar usage and computational issues.
- Thereby reduce undetected computational errors resulting from misunderstanding or misapplication of the mixed Julian/Gregorian? calendar.
- Preserve the mixed Julian/Gregorian? calendar definition within CF, for legitimate purposes.
- Constraint specifications support optional range checking to catch accidental misapplication and computational errors in time coordinates.
6. Status Quo
If no alternative name is provided, the ongoing hidden usage of a complicated calendar will continue to engender misunderstanding, undetected errors, and controversy.
7. Text of Proposed Changes
- In section 4.4.1, Calendar, insert a new definition as the second item in the list of calendars:
Mixed Julian/Gregorian? calendar, with constraints. All usage of negative years, year zero, and transitional dates 1582 October 5 through 1582 October 14 inclusive, is prohibited.
- For consistency, change the term "Gregorian/Julian?" to "Julian/Gregorian?" in three places in section 4.4.1. Do not change the formal excerpt from the UDUNITS man page.
- Add a new paragraph in 4.4.1, just above the third from last paragraph which now begins "The mixed Julian/Gregorian? calendar used by Udunits is explained ...":
The calendar name "Julian/Gregorian?" should be used only (a) in genuine historical data sets having a proper need to span the Julian/Gregorian? discontinuity; and (b) to facilitate backward compatibility in existing data sets.
There have been several discussions and proposals over the years about problems with the mixed Julian/Gregorian? calendar. Shown here is the most recent thread. Please consult the CF archives and related mailing lists for more.
- UDUNITS-1 calendar documentation (see "handling time")
- Gregorian calendar, Wikipedia article with history of the transition between Julian and Gregorian calendars