Correct dates

Corrects the fact that:

$ ddate 1 1 0
returns anything useful at all - Gregorian date 0 doesn't exist.

Henceforth:

$ ddate 1 1 -1
yields:
Sweetmorn, Chaos 1, 1166 YOLD

rather than:
Sweetmorn, Chaos 1, 1165 YOLD

a year early.