For quick scripts, I do all my date math with seconds-since-the-epoch, retrieved with "date", and then convert back at the end. The final date benefits from the latest timezone package update, so mostly I do OK.
This mostly work without problem for past time, but it can be wrong for future time, since timezone/DST may change. When you want 2pm next Sunday, you want 2pm next Sunday local time, not whatever 2pm next Sunday is supposed to be as of now.
