Subject: RE: RECUR issue: DST-change anomalies

From: "Lowde, Chris A" <[email-masked]>
Date: Tue, 20 Mar 2001 18:00:31 -0600

An event such as this should only fire once, since all times should be
converted back to GMT for processing, there will only ever be a single
1:30AM for any given time zone.  Taking the change from GMT-5 to GMT-6 the
first occurrence will be 06:30GMT the second 07:30GMT, thus there is not a
problem.  The big problem occurs when you try to do everything in local
time.  And it gets worse if the local system is on a laptop and travels
between two time zones at 01:45 and resets the system clock.  Now they get
four hits.  If this was an automated stock sale, you have big problems.
The user agent can warn the user and have them pick one time.
Note when the clock moves forward there is an hour that never appears in
local time; 02:00 to 02:59 the CUA should trap this too and move it forward
an hour.

Chris Lowde
Infrastructure Architect
Texaco

-----Original Message-----
From: John Stracke [mailto:[email-masked]] 
Sent: Tuesday, March 20, 2001 12:39
To: calsch WG
Subject: RECUR issue: DST-change anomalies

At the calsch dinner last night, we discussed what happens when
you have a recurring event scheduled for, say, 1:30 AM, when it
falls on the night of a DST change, so that 1:30 AM occurs twice
or not at all.  The sense of the table was that the code
evaluating the RECUR should be naive about it: if 1:30 AM occurs
twice, the event occurs twice; if 1:30 AM doesn't occur, the
event doesn't occur.  Otherwise, you get into trying to guess
what the user means.  The CUA, however, can note when this is
happening, and *ask* the user what they want to do, and adjust
the iCalendar with RDATE/EXDATE/EXRULE (expressing the times in
GMT to disambiguate).

The actual decision on what the evaluation code should do is less
important, but it should be consistent across implementations.
Engines should be predictable, but user-agents should be helpful.

--
/==============================================================\
|John Stracke    | http://www.ecal.com |My opinions are my own.|
|Chief Scientist |=============================================|
|eCal Corp.      |The voices say I'm not schizophrenic, but    |
[email-masked]|they're not sure about you.                  |
\==============================================================/

---

From: "Graham Gilmore" <[email-masked]>
Date: Wed, 21 Mar 2001 11:32:27 -0500

----- Original Message -----
From: "Lowde, Chris A" <[email-masked]>
To: "John Stracke" <[email-masked]>; "calsch WG"
<[email-masked]>
Sent: 20 March, 2001 19:00
Subject: RE: RECUR issue: DST-change anomalies

> time.

    Unfortunately, with a recurrence rule, everything is in local time
whether you want it or not.  If a recurrence is generated from the rule
during your crossover period, there's currently no way to tell which
side of the divide it should be in.  Perhaps a parameter could be added
for properties with DATE-TIME value types which would specify whether to
choose DAYLIGHT or STANDARD time if there's a choice.

> And it gets worse if the local system is on a laptop and travels
> four hits.

    This at least is solvable, since we can know the timezones involved.

    Graham

---

From: John Stracke <[email-masked]>
Date: Thu, 22 Mar 2001 14:46:09 -0600

"Lowde, Chris A" wrote:

> An event such as this should only fire once,

How do you decide which time?

> between two time zones at 01:45 and resets the system clock.  Now they get

No, it doesn't; the event is defined in a particular timezone, not in "local
time".

Of course, you would have a problem if "resets the system clock" does not
involve specifying a timezone.  Reasonable OSes store their system clock in
GMT, and let each process worry about timezone on its own.

> The user agent can warn the user and have them pick one time.

Yes, it should.  However, the UI is separate from the code that expands the
recurrence; that code needs to do something predictable, since it can't ask
the user what to do.

> local time

Yes, I did mention that.

--
/==============================================================\
|John Stracke    | http://www.ecal.com |My opinions are my own.|
|Chief Scientist |=============================================|
|eCal Corp.      |If God had meant us to be in the Army, we    |
[email-masked]|would've been born with green, baggy skin.   |
\==============================================================/