I've been looking at the GregorianCalendar class, and I'm finding that after spending some time getting familiar with the interface, it actually is a decent way of dealing with time. Now I have a special timekeeping application, and I'm wondering about possible extensions to the Calendar Class.
For astronomical purposes, there are apparently calendars that differ slightly from common time. There is Coordinate Time and Terrestrial Time, as well as timekeeping in terms of Julian Days (number of days since some date long ago--i.e., today is julian day 2453811. I don't really understand the science involved in these calendars, but the motivation behind these other calendars is to have some standard notion of time, at least throughout the solar system, that accounts for the relativistic time dialations present in orbital mechanics.
So now I am dreaming of an implementation of Calendar, call it TerrestrialTime, that gives me the smart computation on this timescale if I give it a GregorianCalendar instance.
My application deals with computing and interpreting an ephemeris (a database of planetary motion).