Re: Time to UTC
Re: Time to UTC
- Subject: Re: Time to UTC
- From: "Mark J. Reed" <email@hidden>
- Date: Thu, 7 Jan 2010 12:49:06 -0500
Whups, sent reply only to Doug.
On Thu, Jan 7, 2010 at 12:20 PM, Doug McNutt <email@hidden> wrote:
> UTC (Universal time coordinated) is atomic time based on electronic transitions in atoms.
...as corrected by leap seconds. Uncorrected atomic time is TAI.
> GMT is astronomical time based on rotation speed of Earth.
These days, what you call "GMT" above is actually called by
astronomers and physicists UT0, UT1, or UT2, depending on how much
correction is done on the raw measurements. In scientific circles,
"GMT" is no longer meaningful outside of historical references. The
label has been deprecated since 1972 - in no small part because of the
ambiguity in using it: prior to 1925, 0000 GMT was noon and 1200 GMT
was midnight, a fact which has to be taken into account when
interpreting historical observations. The UTC label has no such
ambiguity.
But that's in scientific use. More generally, the term GMT is still
in legal use in many places, including, I believe, the official
definition of the US time zones, and as the designation of UK time
when Summer Time is not in effect.
The official name for the time zone is UTC, so the function names
should probably be changed, or at least an alias provided. But it
should be noted that most computers don't actually use UTC, because
they don't take leap seconds into account when manipulating historical
dates. Instead, they effectively move the zero point forward in time
by one second every time a leap second is introduced.
> If you set the computer clock using the internet once a day you're surely using atomic time corrected for any previously known leap seconds. That would be
> GMT.
No, that would be UTC. Uncorrected atomic time is TAI.
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| References: | |
| | >Time to UTC (From: Yvan KOENIG <email@hidden>) |