Show time in Calendar before the start of the epoch
The mini-calendar in the side bar of the Calendar view allows scrolling the time for the Month perspective back and forward, but it does not permit showing time before 1st January 1970.
If this restriction comes from CALDAV:min-date-time then show a corresponding message when that limit or CALDAV:max-date-time are reached. Otherwise tweak the calendar to know about negative unix timestamps.