when deciding whether a field is a year field. Assume *anything* longer
than 2 digits (if it isn't a special-case doy) is a valid year.
This should fix the "Y1K" and "Y10K" problems
pointed out by Massimo recently.
Check usage of BC to require a positive-valued year; before just used it
to flip the sign of the year without checking. This led to problems
near year zero.
Allow a 5 digit "concatenated date" of 2 digit year plus day of year.
Do 2->4 digit year correction for 6 and 5 digit "concatenated dates".
Somehow forgot this originally. Guess not many folks use it...
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