Set date for themed calendars without advancing world time

[rsimoes]

Currently if one wants to use a themed calendar, Calendar/Weather will advance the actual world time instead of merely showing a different date. It would be convenient if one could set a base date such that the world time would be treated as an offset of the base rather than the literal year 0 of the calendar. Advancing world time can have game-mechanical ramifications depending on the system or installed modules, so a purely aesthetic view of the date should ideally never alter it.