time for time details

[Moderator, John Quarterman]

From: guy at sun.com (Guy Harris)
Date: Sat, 13 Dec 86 14:14:04 PST

> GMT and UTC are not the same.

However, it's not clear whether the term "GMT", when used in documents
describing the way UNIX handles time, refers to GMT or to the time that
would be kept by a clock set to local British time at some point when
British Summer Time is not in effect and then left to run free.

Since, as you point out, GMT is not readily available from time sources, and
since most hardware and most implementations don't know how to stretch or
shrink seconds, I suspect most implementations definitely don't provide real
live GMT.  In fact, since most hardware doesn't receive any UTC broadcasts,
most implementations don't provide real live UTC, either.  (Some machines
don't even do that great a job at providing *any* sort of
precise-to-the-second indication of current time, given the tendency of
their clocks to drift, or the fact that their clocks are set from somebody's
wristwatch.)

This is all somewhat irrelevant to machines that don't synchronize with UTC
and don't know about leap seconds.  Machines that do synchronize with UTC
will have to worry about whether particular time zones follow UTC or not.
If they insert the leap seconds at the same instant that UTC does, there's
no real problem; if they don't, the offset between UTC and local time
presumably just slowly drifts from being an even number of half-hours.

Volume-Number: Volume 8, Number 68