time for time details

[Moderator, John Quarterman]

From: seismo!nbires!vianet!devine (Bob Devine)
Date: Wed, 17 Dec 86 19:39:53 EST

  This is in response to Ron Tolley's article that appeared in mod.std.unix
last week.  My reply corrects the errors.

Bob Devine

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> GMT and UTC are not the same.

  Yes they are (within a very small delta).  GMT (Greenwich Mean Time)
is maintained by the UK while UTC (Universal Coordinated Time) is maintained
by the International Time Bureau in Paris (BIH).  The WWV* broadcasts in the
US are not exactly UTC but neither is GMT.  However, they are within
nanoseconds of UTC.  WWV (and WWVB and WWVH) are the US's official
distributors of the time according to the US's clocks.

  Some confusion results from the use of "GMT".   In common usage, it
means what time it is in the timezone centered on the Greenwich Observatory
which defines zero degrees longitude.  It also means the official UK
time.  GMT is no longer the global standard for time; UTC is (since 1972).

  UTC is an average of all the contributing countrys' clocks (US, UK,
France, Italy, Japan etc all contribute to UTC).  The change of UTC
to stay close to UT1 (the "spinning earth" time) is through the adding
or subtracting of leap seconds.  BIH makes recommendations for such
leap seconds and it is up to the individual countries to follow them.
I don't know of any case where a leap second recommedation was not
followed by a country for its clocks; it doesn't make sense to disregard them.


> The  following  is a list of leap  seconds  which  have  been  added  to
> Universal Coordinated Time (UTC) in order to keep it relatively close to
> solar time.  Note that with  Greewich Mean Time, such  corrections  were
> made  by  stretching  or  contracting  the  length  of  seconds.  UTC is
> generally available through time standards, GMT not readily available.
> This is data derived from an AP story from May 1985.  No data since then
> is known.  There is also no indication  whether the insertions were made
> in local time or in UTC.  Local time is  assumed.  (Wouldn't  Australia,
> Newfoundland,  and other half-hour-off places have fun with inserting an
> extra second in the middle of a pseudo-random hour.)

  A second is not stretched/contracted for leap second adjustments.  The
selected minute will have 59 or 61 seconds.  There are agreements as to
which minute is selected and the BIH issues its recommendation far in
advance selecting the minute.  Currently, and unless the earth goes wacko,
a second is usually added once a year.

Bob Devine

Volume-Number: Volume 8, Number 73