Bruce Van Allen schreef:
> >Better: "UTC (Temps Universel Coordonne, Universal Coordinated Time) is
BTW, Coordonne is spelled Coordonné in HTML.
> >the current standard time scale. Civil time (the time we all use in
> >day-to-day life) is based on UTC, and generally differs exactly an
> >integer number of hours from UTC, depending on your time zone."
>
> Following others' posts, I had backed off from asserting that UTC is
> the current standard time scale.
Although the other time scales are used, UTC is definitely the standard
time scale, as I understand it. It (or the derived local times) is the
time that we all use, that is on our watches and clocks, it's the time
that is followed by trains and buses and radio/TV stations.
Other time scales are used by specialists, who generally know what
they're doing. (UT1 is used in astronomy, because keeping a telescope
pointing one way depends on following the rotation of the earth; an
atomic time close to TAI is used in the GPS.)
> >> Related time scales include: - GMT (Greenwich Mean Time), in which
> >> each twenty-four hour day has exactly 86,400 uniform seconds by
> >> international convention (the "imperial clock");
> >
> >AFAIK this is not true.
>
> What's not true about it? In the structure of the draft description,
> the main point was to establish the mathematical aspects of GMT,
> namely that "each twenty-four hour day has exactly 86,400 uniform
> seconds." Is this not the case?
Short answer: No. GMT is now more-or-less just the name of a time zone,
and therefore is related to UTC. It has leap seconds at the same time as
UTC does.
Long answer: The definition and status of GMT is unclear.
Literally, it is the local mean time at the Royal Observatory in
Greenwich. However, the RO is neither in Greenwich nor in existence any
longer. The local mean time follows UT0 or UT1, probably; it is related
to the apparent position of the sun.
GMT was declared the official time in Great Britain in 1880 by Act of
Parliament. This Act was never repealed or amended, so GB is still on
GMT. France for example has changed its legislation, and is now on
UTC+0100/+0200.
Which means that the time difference between the UK and France is at the
moment 1 hour and 0.4 seconds.
(One strange consequence is that at the beginning of summer-time (DST),
the clocks in France are put forward from 02:00:00.4 CET to 03:00:00.4
CEST, as the EU directive on summer-time arrangements is expressed in
GMT...)
This whole story is based on the definition of GMT in 1880. In practice,
what is called GMT is just the name of the time zone UTC+0000. It has
leap seconds at the same time as UTC does.
Before 1972, GMT was the same as UT, in practice. Each 24h day had
86,400 seconds, that's true, but they were not uniform in length...
Isn't British law fun?
None of this has to go in the FAQ of course. It's just included so that
you can impress your friends. "Hey, do you know what time it is?
Really?"
> >Better: "GMT (Greenwich Mean Time), the civil time in London (and by
> >extension, in the whole UK, and elsewhere) in winter. Since the
> >introduction of UTC, GMT is equal to UTC."
>
> See next FAQ version.
"- GMT (Greenwich Mean Time), in which each twenty-four hour day was
exactly 86,400 uniform seconds; GMT was historically based on civil
time in London (the "imperial clock"); for time calculations on dates
after the introduction of UTC, GMT is equal to UTC;"
The first sentence ("86,400 uniform seconds") is still wrong; the rest
is OK.
> See next version.
Looks good.
But you might want to include some easier questions to start with, for
example:
Q. Does DateTime handle leap seconds?
A. Yes
Q. What is a leap second, anyway?
A. One rotation of the Earth (1 day) is not exactly equal to 24*60*60
seconds. This means that an extra second, numbered 23:59:60, has to be
inserted at the end of a day once every few years. This is similar to
the system of leap days, which account for the difference between 1 year
and 365 days.
Many computer programs are unable to cope with them correctly; DateTime
is!
Q. Tell me more! This sounds like fun!
A. Well, the DateTime modules are based... etcetera
Eugene
