From: Andrew Brown [mailto:atatat@atatdot.net] Sent: Tuesday, July 29, 2003 12:20 AM
it struck me that the tz data base would not be an unreasonable place to store the "local" date at which the gregorian reform took place
I privately started a little list to do that, but gave it up for lack of time. I found it hard to get authoritative data. It is hard to get reliable information about transitions that occurred that long ago.
italy adopted the reform (1582/10/05),
That's true for most of Italy, but Toke Norby writes that Florence and Pisa didn't switch until 1749-12-21 Julian (1750-01-01 Gregorian).
or japan (1872/12/20).
You're giving a Julian date, but didn't Japan use a lunar calendar previously? And I've seen different sources about when the actual change occurred, ranging from 1873-01-01 Gregorian (your date) through 1919-01-01. Possibly they switched for some purposes at first but not others, which would mean there's no single date of switch.
(modulo the various cantons in switzerland and the parts of germany that were protestant in the 16th century).
It's worse than that. In some of those places the date depended on who you were talking to. Even in the same town, part of the population would use Julian and the other part Gregorian. Also, once you go back that far, you should also address the issue of when the year started. For example, much of Europe started the calendar year on March 25 from the 12th through the 16th centuries. And some of Europe started the calendar year on Christmas Day. Anyway, if I haven't discouraged you from doing the job right (:-) here are some references that you may find of interest: <http://www.norbyhus.dk/calendar.html> <http://www.tondering.dk/claus/calendar.html> Two good sources for general calendrical information. <http://www.genfair.com/dates.htm> Explains calendrical issues for western Europe particularly well. Oriented towards genealogists. <http://www.calendarists.com> By far the highest-quality source of information for computerized calendars. However, they don't worry about transition dates much.