Daylight Savings

Monty Solomon monty at roscom.COM
Sat Apr 20 02:21:15 UTC 1996

FYI.  Several excerpts from RISKS DIGEST 18.02, 18.03, 18.04, 18.05 ...


Date: Sun, 7 Apr 1996 13:31:38 -0400
From: Matt Welsh <mdw at CS.Cornell.EDU>
Subject: De facto Daylight Savings

At there is a brief discussion
of the rules for Daylight Savings Time changeovers for central Europe
and the UK. At the end of the page it says:

> NOTE: From autumn 1996 the rule of changing from standard time to
> daylight time is changed. The new rule is valid for central Europe
> including the UK is:
> Standard time    to  Daylight Saving       LAST SUNDAY OF MARCH
> Daylight Saving  to  Standard time         LAST SUNDAY OF OCTOBER
> The rule is a "de facto standard," not a law.
> The switching occurs at 01:00 UTC for central Europe (Stockholm Paris etc.)
> Local time that is at 02:00 STD to DST  and  03:00 DST to STD.
> (STD = Standard time, DST = Daylight Saving Time or Summer Time.)
> Note: The legal switching is steered by laws that state dates for a couple
> of years. When a period ends a new law is issued that gives the dates for
> the next years.
> The Laws do not state any general rule. Only dates for a couple years
> each time. The "de facto rule" works, but there is no warranty it will
> work forever!

With all of our scrambling about to deal with the Year 2000 problem,
shouldn't we be just as concerned with this inconsistency that arises
yearly (especially if there are no 'hard and fast' laws/standards to dictate
DST changeovers)?

M. Welsh, mdw at


Date: Wed, 10 Apr 1996 11:31:07 -0600 (MDT)
From: wampler at
Subject: Daylight Savings Time problem

I was hit by a daylight savings time problem Monday, the day after the time
changed here. My machine is running four different operating systems:
Windows 95, Windows NT, OS/2, and Linux. Since I'd doing cross platform
development, I usually boot at least two different OS's a day.

Monday, I booted Windows 95 first. At startup, I was greeted by a polite
messages asking if the time should be changed to DST.  Fine. Time changed
and correct.

Later in the day, after booting both NT and Linux, I noticed the time was
yet another hour ahead. Either NT or Linux (and I suspect NT, but can't
confirm that) had also, but invisibly, changed to DST.

After some thought, and a class discussion in the software engineering class
I teach, I've concluded this is not an easy problem to solve. In this case,
there were two basic contributing factors I can figure out. First, PCs keep
the internal clock in local time. Not a good idea -- it should be Universal
Time -- but reality. The problem is then that NT or Linux made the assumption
it was the only OS on the machine, and was free to update the time. Unlike
Win95, which could be polite about the change because it is normally a
single user system, NT and Linux both could reasonably assume they don't
normally get shut down each evening, and thus the silent time update (I'm
guessing). It would be unreasonable to expect confirmation from an operator.

In this case, however, the time update did come at boot time. It seems to me
a better update policy for NT/Linux would be to silently update the time if
the change happened while running, and require a confirmation if it happens
at boot time. Not perfect, but better. I tried OS/2, also, and it just
ignored the time change.

Bruce E. Wampler, Ph.D., Adjunct Professor, Department of Computer Science,
University of New Mexico   wampler at


Date: Tue, 9 Apr 1996 22:08:02 GMT
From: Dik.Winter at (Dik T. Winter)
Subject: Re: De facto Daylight Savings

 > At there is a brief discussion
 > of the rules for Daylight Savings Time changeovers for central Europe
 > and the UK. At the end of the page it says:
 > > The rule is a "de facto standard," not a law.
 > With all of our scrambling about to deal with the Year 2000 problem,
 > shouldn't we be just as concerned with this inconsistency that arises
 > yearly (especially if there are no 'hard and fast' laws/standards to dictate
 > DST changeovers)?

But there *are* 'hard and fast' laws that dictate DST changeovers.  There is
however *not* a law that dictates this far into the future but only for a
few coming years.  (Note that these laws, EU directives, are made up far in
advance.  It was already known a few years ago that the EU would change the
rule this year.)  It appears not to be very advisable to cast changeover
dates in concrete.  It is up to the software to deal with this flexibility
and Andrew Olson's timezone package deals very well with it.  (And some
software does not handle it well at all.  Most annoying was a bug in SGI's
software which thought the last Sunday in September last year was October 1,
and so switched out of DST one week late.  Exactly the same bug stroke again
this time when the software thought that the last Sunday of March was March
24, and so switched into DST one week early.  Surprising that the bug was
not fixed in that half year.)

dik t. winter, cwi, kruislaan 413, 1098 sj  amsterdam, nederland, +31205924098
home: bovenover 215, 1025 jn  amsterdam, nederland;


Date: Thu, 11 Apr 1996 18:45:14 -0400
From: "John F. Whitehead" <jfw at>
Subject: Re: Daylight Savings Time problem: Netscape 2.* reload

There has been another side effect of the daylight savings time change,
with the Netscape Navigator browser: caches have incorrect times and no
longer work properly for documents that change frequently.

Netscape Navigator version 2.x for Windows and Unix platforms is an hour off
in its cache-file handling.  If a user tries to reload a page that has
changed within the last hour, the browser still thinks its cached version is
more up to date and won't retrieve the new version.  (After an hour, this is
no longer an issue.)  This has been a problem with news organizations (such
as ours), chat/bulletin boards, and java applets that need to be updated

Netscape's "force reload" procedure (shift key + reload button) doesn't
always work either: the only solution is to flush the memory and disk caches
before a reload, or to set them to size zero.  Netscape has made no official
statement, but apparently has said the bug won't be fixed in the next
version of the software (2.1) but in the one after that (3.0 (Atlas)).

The Macintosh version does not suffer from this bug, nor do versions 1.x, or
browsers from other manufacturer.

The risk (aside from the obvious one related to programming for time
changes) is trusting that a market-leader software company is going to have
bug-free software.

John F. Whitehead  OnLine Technical Director
919-821-8605  jfw at

  [This problem was also reported by CurtAkin at and
  Prentiss Riddle <riddle at> -- next message.  PGN]


Date: Mon, 15 Apr 1996 09:14:54 -0500 (CDT)
From: Prentiss Riddle <riddle at>
Subject: Another Daylight Saving Time problem: Netscape 2.* reload

[...] One workaround is said to be to run Netscape in California time, e.g.
under Unix:

        setenv TZ PDT ; netscape &

Defying the RISKS tradition of intoning that "the risks are obvious",
one could draw the following lessons:

   -- Time-dependent functions should be tested using multiple time
      zones and both DST- and non-DST dates.

   -- In networked applications, local time issues can cause more than
      just local problems.

Prentiss Riddle <riddle at>
RiceInfo Administrator, Rice University


Date: Thu, 11 Apr 1996 16:20:49 -0400 (EDT)
From: beaton1 at (Lorne Beaton)
Subject: Another Daylight Savings Time risk: billing

My university recently (ca. January 1, after a testing period of about a
month) instituted a new dialup service, for which students and faculty are
charged 75 cents per hour peak and 60 cents off-peak.  A couple of days ago
I logged on and saw something like the following in my logon message:

>  Charges THIS month to date   500 minutes for a cost of $   49.59
>  Charges LAST month (total)  2089 minutes for a cost of $   24.76

(Note that these aren't the exact figures, but you get the idea.)  Needless
to say I was consternated.  After complaining to the admins, I learned that
the billing discrepancy arose from the change to Daylight time.  Night owl
that I am, I happened to be logged in at the exact moment that 2:00 a.m. EST
became 3:00 a.m. EDT.  Needless to say, this was the first time they had
dealt with the changeover.  Happily, the problem has since been fixed, but
the risk is self-evident.

Lorne Beaton <beaton1 at>


Date: Tue, 16 Apr 1996 10:23:50 -0700
From: phaedrus at (Mark Phaedrus)
Subject: Re: Daylight Savings Time problem: Netscape 2.* (Whitehead, R 18 04)

     Actually, Netscape has publically announced that "We are preparing to
release Netscape Navigator 2.02 in the next two weeks to fix this problem."
(Source: <URL:>.)  They also give
one other, rather unsatisfactory workaround (besides clearing or zeroing
your disk cache, or setting the time-zone variable correctly): set your
machine's clock back an hour...

  [Also noted by
     Prentiss Riddle <riddle at> and
     "J. David Stanton, Jr." <jstanton at>.  PGN]


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