Dumb questions about GMT offset in tm structure

Robert Elz seismo!munnari!kre
Sat Mar 14 22:10:15 UTC 1987

    Date:        Sat, 14 Mar 87 13:29:55 EST
    From:        elsie!ado (Arthur David Olson)
    Message-ID:  <8703141829.AA01740 at elsie.UUCP>

    Dumb question #1:  what applications use the offset from GMT of local time?
Anything which wants to record time in local time (for ease of
human reading) but be precise about the zone so the precise GMT
time can be reconstructed.  Mailers are a prime example, the
Date: line should contain such an offset (only in the 5 US
zones is it allowable to use an abbreviation .. except the
stupid US military one letter abbreviations; and even those
5 are only allowed in rfc822 style mail, ISO stuff always
requires the numeric offset)

    Dumb question #2:  if there's a way to turn tm structures ...
    does .. [ being able to deduce it ] ...
    obviate the need for a GMT offset element in the tm structure?

Certainly, this is possible.  Initially I wasn't realy impressed
with the need for the offset in the struct tm because of how easy
it is to obtain this way (even without the inverse functions, its
not hard to do a struct subtraction (ie: multiple subtractions,
with additions & multiplications to combine the answers) of
the results of gmtime() and localtime()).  But the zone offset
information is trivially available to localtime, it seems like
it can't hurt to provide this extra information, essentially
for free.


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