UTC as basis for time legislation

Clive D.W. Feather clive at davros.org
Mon Sep 26 14:13:36 UTC 2011

Tobias Conradi said:
>> No, the law here says legal time is GMT, but most official time signals
>> are UTC. There have been a few unsuccessful attempts to deal with the
>> mismatch between de jure and de facto UK time, e.g.
>> http://www.publications.parliament.uk/pa/ld199798/ldhansrd/vo970611/text/70611-10.htm
> So there is one flavor of GMT that is UTC and there is legal GMT which
> is something different.

Yes and no.

Legal time in the UK is Greenwich mean time (or that plus one hour). That
means mean solar time at Greenwich, which is equal to UT1 within a few
milliseconds (it depends on exactly what you mean by "mean" and exactly
where "Greenwich" is).

The easiest-to-find sources of time in the UK are all UTC. To actually get
your hands on GMT you need a source of DUT and to do the subtraction. That
does *not* mean that "there is one flavor of GMT that is UTC".

I've not found a reported law case where the difference is explored, let
alone where it mattered. The closest I got was a case hinging on a time
difference of 8 seconds between two events, but there was no mention of
which of GMT or UTC applied.

> 2011i/tzdata equates GMT for 2011 with UTC.

In effect, yes. It takes the view that the difference is too small to
matter *in this context*.

>>> So if country A says its legal time is an hour ahead of GMT and B says
>>> they are an hour ahead of UTC, those are technically different statements.

> To my understanding the statement of country A is ambiguous. Whether
> there is approximation in tzdata depends on how one interprets the the
> statement of country A.

No. The statement of country A is *not* ambigous (except at the millisecond
level as mentioned above). The statements are technically and de jure
different. Whether country A actually meant UTC when it wrote GMT is a
separate question that you'd have to ask country A.

> This has nothing to do with ITU-R TF.460-6.

But the proposed amendment to it would mean that "too small to matter in
this context" will cease to be true eventually.

Clive D.W. Feather          | If you lie to the compiler,
Email: clive at davros.org     | it will get its revenge.
Web: http://www.davros.org  |   - Henry Spencer
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