epochs and the gregorian reform

[Paul Eggert]

Andrew Brown <atatat at atatdot.net> writes:

> here's a list of points i think we could probably agree on.
>
> (1) things changed.
> (2) having a list of said changes would be good.
> (3) assembling such a list will be difficult.
>
> actually, the third point would probably be about as hard as it was to
> bootstrap the tz data, maybe simpler,

I'm not sure that I agree.  The tz data was bootstrapped by ignoring
most problems that occurred before 1970, which is the vast majority of
the problems.  People interested in historical use of calendars have
no such luxury.

> and we also don't have lots of "crazy" governments running around
> saying "gregorian!  julian!  reformed julian!  gregorian!", so the
> churn ought not to be as high.

That problem is relatively limited now, as most people know and use
the Gregorian calendar even if they are in a location (Iran, say?)
where another calendar is official.  However, the point of coming up
with such a list will be to go back into history, where the problem
was very real.  Anyone who seriously wants to date events in the
Middle Ages will know what I'm talking about: at times it seems that
every author used a different calendrical method.  And it's not just
the Middle Ages: for example, three different dating methods were used
in the logs of Captain Cook's voyages, depending on whether one
thought the day started at 00:00, 12:00, or 24:00.

It would take a lot of work to come up with a reliable history of
calendrical usage.