Support Gregorian dates from 1582 to 1970

[hollasch]

See https://en.wikipedia.org/wiki/Gregorian_calendar.

The Gregorian calendar was enacted 1582-10-15. Add code to support such dates. For delta display, we have room to use negative 64-bit values. This task is made easier by the fact that leap seconds didn't exist until 1972, so the calendar is predictable for that time period.

For non-delta values, this may not be simple.