Precise timekeeping for dates over 100 years old is not usually required
Agreed.
though perhaps you work in an area that is an exception?
Thankfully, no. These days I rarely have to deal with dates at all. I still have the scars though..
I find the anomalies extremely interesting for their own sake.
They aren't entirely gone in the modern world, either. Mount Athos is one of the weirder ones, but Ethiopia still uses a calendar similar to the Julian calendar[1].
More practically, though I suspect that in almost all places with non-conventional calendars you can probably use a normal Gregorian calendar too and people will convert for themselves.
1. Precise timekeeping for dates over 100 years old is not usually required (though perhaps you work in an area that is an exception?)
2. UNIX timestamps are independent of timezones and can be easily mapped to and from local time by standard, well-tested libraries. The Olson database maintains tables of local time zone information for modern time zones: http://en.wikipedia.org/wiki/Tz_database
Here is some discussion of Mount Athos, not sure if this was ever implemented or not though: http://comments.gmane.org/gmane.comp.time.tz/1610