"Cub Driver" wrote in message
...

Boston is always GMT-4 or -5, and there are only two
dates when the variable comes into play.

Of course planes depart London on local time, not Zulu.

The U.S. & Britain don't change summer/winter times necessarily on the
same date. (I think the gap is most noticable in autumn.)

No, I meant that there are only two dates when the variable comes into play
for each station. You run the whole thing in GMT, from the computer's pov.