Flights that Cross Midnight

I'm not sure where to post this, so please re-direct me if there is a
better place.

I have a project where I have to determine the correct GMT time offset
for the arrive staton of a flight. I'm doing this for a company that
deals with airline carriers all over the world. All that I have to
work with are the depart station, depart station local time, arrive
station, arrive station local time, and the flight effective date
(which is the date the flight departs). The GMT data that I work with
is in local times per the station.

Some thoughts or questions to consider . . .

In order to get the correct GMT offset for the arrive station, I have
to know the correct date to use. This usually is not a problem,
except it can be if a flight crosses over midnight (meaning it will be
the next day at the arrival station) and the flight departed the day
before time change date.

Their current function always gets the GMT based on the effective date
of the flight. This is fine for 99% of the time. When a flight
crosses midnight and it's around time change dates, the effective date
of the flight might need to have a day added to it if the flight
crossed over midnight.

Any input or direction would be greatly appreciated.