"ydm9" wrote in message

If I can determine what date
to use to get the arrive station GMT offset, I'll have it made.

I would think it would make more sense to input the GMT correction, then
calculate any cross-midnight adjustment based on the flight arrival time.
The GMT offset will be a constant (+/- 1 if that locale uses DST) regardless
of your arrival time. Boston is always GMT-4 or -5, and there are only two
dates when the variable comes into play.