pytz question: GMT vs. UTC

[Grant Edwards]

On 2014-01-29, Skip Montanaro <skip at pobox.com> wrote:
> According ato the pytz doc (http://pytz.sourceforge.net/):
>
> "UTC is Universal Time, also known as Greenwich Mean Time or GMT in
> the United Kingdom."
>
> If they are equal,

The question is _are_ they equal?

There is an exact defintion for what "UTC" is, and there's another
exact definition of what UT1 is (more about this later).  Civil
timezones are defined as offsets from UTC.

It seems that "GMT" no longer has an exact definition (at least from a
metrologist's perspective) can be used to mean either UTC or UT1.  UTC
and UT1 can differ by up to 1 second.  Leap seconds are occasionally
added to UTC to keep it from drifting more than 1 second from UT1. 

>From a metrology point of view, what was originally called "GMT"
(solar time at 0 degrees longtitude) is now called "UT1".  So some
people rightly claim that "GMT" means UT1. But nobody actually _uses_
UT1 (except metrologists and astronomers).

All civil time is based on UTC: the official time in Greenwich (except
during BST) is not UT1, it's UTC.  So some other people rightly claim
that "GMT" refers to UTC.

In a software libary context, I would say that GMT should mean UTC and
they ought to be considered equal and should always produce identical
results.  In a metrology context, people saying "GMT" probably ought
to be smacked across the knuckes with a 12-inch platinum-iridium ruler
and asked to try again until they specify either UTC or UT1 (or some
other precisely defined UT-flavor).

http://en.wikipedia.org/wiki/Universal_Time

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Grant Edwards               grant.b.edwards        Yow! A dwarf is passing out
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