[ntp:questions] Leap second functional question

Danny Mayer mayer at ntp.isc.org
Sat Feb 23 02:40:52 UTC 2008

Kevin Oberman wrote:
>> Date: Fri, 22 Feb 2008 17:55:04 -0500
>> From: Danny Mayer <mayer at ntp.isc.org>
>> Sender: questions-bounces+oberman=es.net at lists.ntp.org
>> David Woolley wrote:
>>> Evandro Menezes wrote:
>>>> Aren't you confusing UTC and GMT?  Or maybe I'm the one confusing
>>>> both...
>>> Nearly everyone confuses UTC and GMT.  GMT is an obsolete name for UT1, 
>>> or something close, but the BBC still uses it to, incorrectly, mean UTC.
>> That's actually not true. GMT exists in the UK as the local time zone 
>> reference name. GMT was renamed UTC for mostly political reasons. GMT 
>> follows UTC but it isn't an incorrect reference.
> Are you really sure? I've always read that GMT is UT1, not UTC. I just
> read the article on wikipedia and it seems to agree that GMT is UT1.

The official standards body for this in the UK is the National Physics 
Laboratory so that's where you should look. The general reference is 
here: http://www.npl.co.uk/server.php?show=nav.275 Specifically you 
should look at the following PDF: 
http://www.npl.co.uk/upload/pdf/Timescales_ADDED.pdf which specifies 
that "GMT remains the legal basis of the civil time for the UK." and 
"adopted the term Universal Time (UT) for the "new" GMT." Interesting 
enough it says UT and not UTC or UT1. The next paragraph talks about the 
UT% types and how UT1 is the only one in widespread use but does not say 
that GMT uses it.

The following document does say that GMT is UT1:
"In effect, the length of the seconds of Universal Time (UT1, as GMT is 
now officially known) varies slightly to keep in step with the changes 
in the Earth's rotation."

I do know that the BBC inserts a leap second when needed (I've heard 
them to it - an extra long pip within the regular 6 pips.

I stand corrected.

> Of course, UT1 is always within 1 second of UTC.
> In any case, the definitions in the various Wikipedia articles (UTC, UT,
> and GMT) all agree with what I learned dealing with timing issues in the
> past, although both those folks (the ones in Boulder, Colorado) and
> Wikipedia could be wrong.
> The article also states that UTC does not always have 86,400 seconds in a
> day, although POSIX specifies that a day is always 86,400 seconds long.
> If anyone can provide a reference from one of the real standards
> specifications, I would appreciate it. (If it does not match with the
> information I have received form the NIST folks in Boulder, I'll be very
> surprised.)

See above for the UK.


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