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From: mcma...@netcom.com (Dave Mc Mahan)
Subject: How to determine when daylight savings time switches?
Message-ID: <mcmahanCrJt3v.421@netcom.com>
Followup-To: poster
Keywords: daylight savings time
Organization: Dave McMahan @ NetCom Services
Date: Fri, 17 Jun 1994 15:45:30 GMT
Lines: 18
I have been asked to find out how to determine the exact date used to cause
a switch from standard time to daylight savings time. I know that there
are two such switches each year (one in the fall to switch to standard time
and one in the spring to switch to daylight savings time) and I know that
the switch always occurs early on a Sunday morning, but I need to know how
those particular days are selected. Can you help me?
If there are places that pick the date to switch in non-standard ways, I
would like hear about those as well.
Please send all replies directly to the poster (mcma...@netcom.com), since
this is being put into several newsgroups at once.
-dave
--
Dave McMahan mcma...@netcom.com
37N 17.382 121W 59.190
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From: mou...@watson.ibm.com (Joe Seigh)
Subject: Re: How to determine when daylight savings time switches?
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Date: Wed, 22 Jun 1994 15:31:43 GMT
Disclaimer: This posting represents the poster's views, not necessarily those
of IBM.
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Keywords: daylight savings time
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The heck with daylight savings time. What I want to know is how that
leap second that they are supposed to tack on to the end of June works. None
of the unices I've looked at seem to be able to handle it. Basically
I think you have to decrement a second from the epoch which means a
defacto moving the definition of start of the epoch up a second.
BTW, when is the second added, local time or GMT?
Joe Seigh
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From: Erik Naggum <e...@naggum.no>
Newsgroups: comp.realtime,sci.geo.satellite-nav,comp.programming,netcom.general
Subject: Re: How to determine when daylight savings time switches?
Date: 22 Jun 1994 18:54:41 UT
Organization: Naggum Software; +47 2295 0313
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[Joe Seigh]
| What I want to know is how that leap second that they are supposed to
| tack on to the end of June works. None of the unices I've looked at
| seem to be able to handle it. Basically I think you have to decrement
| a second from the epoch which means a defacto moving the definition of
| start of the epoch up a second.
have you attempted to touch a file at 23:59:60? will it still read 23:59:60?
| BTW, when is the second added, local time or GMT?
Coordinated Universal Time (UTC). GMT doesn't _have_ leap seconds.
</Erik>
--
Erik Naggum <e...@naggum.no> <S...@ifi.uio.no> | memento, terrigena
ISO 8652 Ada/ISO 8879 SGML/ISO 9899 C/ISO 10646 UCS | memento, vita brevis
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From: unr...@cd4680fs.rrze.uni-erlangen.de (Markus Kuhn)
Newsgroups: comp.realtime,sci.geo.satellite-nav,comp.programming,netcom.general
Subject: Re: How to determine when daylight savings time switches?
Date: Fri, 24 Jun 1994 13:00:51 +0200
Organization: Regionales Rechenzentrum Erlangen, Germany
Message-ID: <2ueed3E2vj@uni-erlangen.de>
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<19940622.3190@naggum.no>
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Erik Naggum <e...@naggum.no> writes:
>GMT doesn't _have_ leap seconds.
Wrong.
GMT is London winter time, and it is defined to be identical to UTC, so of
course if UTC jumps, so does GMT. Perhaps you mean that "mean solar time" and
UT0 don't have leap seconds? Yes, that's right. Without huge comet impacts,
the earth won't jump a second.
Recommended reading: IERS (International Earth Rotation Service) Bulletins
and RFC 1305. There you'll also find references to non-ftpable literature.
(e.g. ftp.uni-erlangen.de:pub/IERS/).
E.g. from RFC 1305 a very brief introduction to time:
------------------------------------------------------------------------
Determination of Time and Leap Seconds
The International Bureau of Weights and Measures (IBWM) uses
astronomical observations provided by the U.S. Naval Observatory and
other observatories to determine UTC. Starting from apparent mean solar
time as observed, the UT0 timescale is determined using corrections for
Earth orbit and inclination (the Equation of Time, as used by sundials),
the UT1 (navigator's) timescale by adding corrections for polar
migration and the UT2 timescale by adding corrections for known
periodicity variations. While standard frequencies are based on TAI,
conventional civil time is based on UT1, which is presently slowing
relative to TAI by a fraction of a second per year. When the magnitude
of correction approaches 0.7 second, a leap second is inserted or
deleted in the TAI timescale on the last day of June or December.
For the most precise coordination and timestamping of events since 1972,
it is necessary to know when leap seconds are implemented in UTC and how
the seconds are numbered. As specified in CCIR Report 517, which is
reproduced in [BLA74], a leap second is inserted following second
23:59:59 on the last day of June or December and becomes second 23:59:60
of that day. A leap second would be deleted by omitting second 23:59:59
on one of these days, although this has never happened. Leap seconds
were inserted prior to 1 January 1991 on the occasions listed in Table
8 (courtesy U.S. Naval Observatory). Published IBWM corrections
consist not only of leap seconds, which result in step discontinuities
relative to TAI, but 100-ms UT1 adjustments called DUT1, which provide
increased accuracy for navigation and space science.
------------------------------------------------------------------------
Markus
--
Markus Kuhn, Computer Science student -- University of Erlangen,
Internet Mail: <msk...@cip.informatik.uni-erlangen.de> - Germany
WWW Home: <http://wwwcip.informatik.uni-erlangen.de/user/mskuhn>
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eggert
From: egg...@twinsun.com (Paul Eggert)
Subject: Re: How to determine when daylight savings time switches?
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<19940622.3190@naggum.no> <2ueed3E2vj@uni-erlangen.de>
Date: Sat, 25 Jun 1994 01:17:42 GMT
Lines: 17
unr...@cd4680fs.rrze.uni-erlangen.de (Markus Kuhn) writes:
> Erik Naggum <e...@naggum.no> writes:
> > GMT doesn't _have_ leap seconds.
> Wrong. GMT is London winter time,
You're both right. The problem is that there are two disagreeing
interpretations of the term ``GMT'': the official definition ``mean
solar time observed at Greenwich'' used by Naggum, and the unofficial
meaning ``time used by the BBC World Service'' used by Kuhn.
Since nobody has really used the official definition of GMT for decades,
pedantic timekeepers avoid saying ``GMT'' these days;
it avoids confusion and prevents arguments.
By the way, ``London winter time'' was UT+0100 from 1940 through 1945,
and from 1968 through 1971, so in that sense, at least, Kuhn is incorrect.