Great database work!
However, a nitpick: there is no year zero, in either Gregorian or Julian calendars. The years go ...2BC, 1BC, 1AD, 2AD, ... This is a pain for mathematicians.
The artificial proleptic Gregorian calendar that no-one ever used does have a year zero.
Of course, zero is a point with no duration on the timelines!
Best wishes, Chris