JRS: In article <
[email protected]>, dated Fri, 31
Dec 2004 12:22:24, seen in Mark Preston
The wise reader will also know that GMT and UTC are the same thing.
Unfortunately, as we all know, wise readers are as rare as rocking-horse
droppings...
No, GMT and UTC are not the same thing, even apart from what I was
referring to, which is that a coder must allow for the possibility of
getting either string from a Date Object method.
Javascript treats them as synonymous, and implements time as GMT; few
computers have the information needed to implement UTC.
GMT has 24 * 60 * 60 seconds in EVERY day [@], but the seconds are of
slightly varying length so that, on average, GMT Noon is at mid-day at
Greenwich.
UTC has 24 * 60 * 60 SI seconds in most days, and the seconds are of
constant length[+], but occasionally there can be one added or omitted,
so that, on average, UTC Noon is at mid-day at Greenwich.
Those are a sort of average mid-day - look up "Equation of Time".
The difference between UTC and GMT is kept under, IIRC, 0.9 seconds.
Above, UTC means UTC; but IIRC GMT above may mean some other timescale,
perhaps UT, with similar properties - GMT itself being now deprecated.
See <URL:
http://www.merlyn.demon.co.uk/leapsecs.htm>, etc.
The above is not precision-grade; for that, refer to such as NPL, NIST,
and "The International Earth Rotation Service (IERS)" at
<URL:
http://hpiers.obspm.fr/>.
The advantage of GMT, as a term, is that it is generally interpreted as
needed, with days each containing 86400 parts; as opposed to UTC, where
each day contains 86400+-1 SI seconds. Javascript has no support for
Leap Seconds. It accepts new Date("2005/06/30 23:59:59 UTC") here as
nearly Jul 1 0100h, but considers new Date("2005/06/30 23:59:60 UTC")
as meaning NaN, whereas that second *may* occur.
Be aware that, unless recently changed, UK legal time is GMT, but time
signals here are UTC. The cautious drinker will prudently drain his
glass at least 0.9 seconds before the stipulated moment seems to appear;
and it would be interesting to argue on such a basis about a marginal
case of Ken Livingstone's congestion charge.
[@] Except 1924 Dec 31 GMT or 1925 Jan 1 GMT.
[+] La seconde est la duree de 9 192 631 770 periodes
de la radiation correspondant a la transition entre les deux
niveaux hyperfins de l'etat fondamental de l'atome de cesium 133
(CGPM 13, 1967, Resolution 1). [Francophones add accents to taste.]
