In my old job, I was once asked if it is possible in the United
States to say in which time zone an address is located from the state
or county it is in. Normally, this is possible, since time zone
boundaries normally coincide with state boundaries, and when not,
then with county boundaries. I know of one case, however, where the
time zone boundary runs straight through a county: Gulf county in
Florida (and we all know Florida is in two time zones since the
presidential elections, don't we). My question is: how is this
boundary defined? Is there some quarternary (sub-county) boundary
that is used as time zone boundary here? Can you tell, for example,
the time zone from the ZIP code of an address?

I read somewhere that for election purposes only, as the opening and
closing time of the ballot boxes has to be the same throughout a
county (at least in Florida, that is), Gulf county uses the same time
throughout the county. So in one part the voting starts at 6 a.m.,
and in the other part at 7 a.m.

The splitting up, time-wise, of Gulf co., has to do with the railway
that runs through it (if I recall correctly). After all, the whole
implementation of the time zone system was really initiated by the
railways in the 19th century.

Are there any other cases like this in the US? Or in other countries?
There must be.

Peter S.