std-unix@ut-sally.UUCP (12/13/86)
From: utah-cs!hplabs!hpfcla!hpfclj!hpfcdg!rgt (Ron Tolley) Date: Thu, 11 Dec 86 18:16:53 est GMT and UTC are not the same. 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. 1) 1972 June 30 23:59:60 2) 1972 June 30 23:59:61 3) 1973 June 30 23:59:60 4) 1974 June 30 23:59:60 5) 1975 June 30 23:59:60 6) 1976 June 30 23:59:60 7) 1977 June 30 23:59:60 8) 1978 June 30 23:59:60 9) 1979 June 30 23:59:60 10) 1981 June 30 23:59:60 11) 1982 June 30 23:59:60 12) 1983 June 30 23:59:60 13) 1985 June 30 23:59:60 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.) This information has been pieced together from scattered sources. I reserve the right to be proven wrong. Ron Tolley Volume-Number: Volume 8, Number 67
std-unix@ut-sally.UUCP (Moderator, John Quarterman) (12/23/86)
From: guy@sun.com (Guy Harris)
Date: Sat, 13 Dec 86 14:14:04 PST
> GMT and UTC are not the same.
However, it's not clear whether the term "GMT", when used in documents
describing the way UNIX handles time, refers to GMT or to the time that
would be kept by a clock set to local British time at some point when
British Summer Time is not in effect and then left to run free.
Since, as you point out, GMT is not readily available from time sources, and
since most hardware and most implementations don't know how to stretch or
shrink seconds, I suspect most implementations definitely don't provide real
live GMT. In fact, since most hardware doesn't receive any UTC broadcasts,
most implementations don't provide real live UTC, either. (Some machines
don't even do that great a job at providing *any* sort of
precise-to-the-second indication of current time, given the tendency of
their clocks to drift, or the fact that their clocks are set from somebody's
wristwatch.)
This is all somewhat irrelevant to machines that don't synchronize with UTC
and don't know about leap seconds. Machines that do synchronize with UTC
will have to worry about whether particular time zones follow UTC or not.
If they insert the leap seconds at the same instant that UTC does, there's
no real problem; if they don't, the offset between UTC and local time
presumably just slowly drifts from being an even number of half-hours.
Volume-Number: Volume 8, Number 68std-unix@ut-sally.UUCP (Moderator, John Quarterman) (12/23/86)
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 ------------------------------------------------------------------ > 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
std-unix@ut-sally.UUCP (Moderator, John Quarterman) (01/08/87)
From: ames!pyramid!nsc!nscpdc!nscpdc.nsc.com!djg Date: Sun, 28 Dec 86 11:47:06 pst > 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. ..... > > 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. .... > A second is not stretched/contracted for leap second adjustments. The > selected minute will have 59 or 61 seconds. Note as above it was G.M.T that was stretched. I used to work at the R.G.O. Whenever possible a zenith tube reading of Polaris was used (up to the installation of caesium clocks 20? years ago) to correct an oscillator defining a 10MHz signal sent via land line the Rugby time centre for broadcast. On every hour the signal was inverted 5 seconds before the hour to synchronise clocks (This is still done but from atomic clocks). Since using U.T.C the leap seconds are manually added or subracted at the appropriate time (Yes someone at midnight dec 31 has to go down to the time computer and press a button). Note that since UTC is the calculated best fit of many atomic clocks it can never be given in "real time" but only after the fact. (Most laborities(observatories) use quartz clocks set against a reference and then post-calibrate it). Volume-Number: Volume 9, Number 9