Of course, IBM's official answer to this question is that querying arsdate is the ONLY true method of calculating the 'arsdate' value, which is a good answer when it's feasible.
The basic function is number of seconds since midnight January 1st, 1970, divided by 86400 (ie, the number of seconds in a day).
You must perform your own time-zone calculations, and there is no accommodation for leap-seconds as far as I know.
Good luck -- and please share your solution!
-JD.