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Re: How to tell (in advance) if a date-time is ambiguous?

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From:
Dave Rolsky
Date:
July 10, 2017 18:20
Subject:
Re: How to tell (in advance) if a date-time is ambiguous?
Message ID:
CAHKw1MJ-vOk-jVjuFRp5dj_Aamjv0NKOHCmehVHtkuxj+uev5Q@mail.gmail.com
While you could in theory write code that would be correct for all past
datetimes, the future doesn't work the same way. As Eric noted, time zones
are political. I have seen DST transitions altered with mere days (or
less!) notice given. This means that anything you determine about the
future could be wrong.

If you're trying to avoid these, the best advice I could give would be to
avoid the 12am-4am window, which AFAIK is when most (all?) transitions have
occurred historically.


Cheers,

Dave Rolsky
http://blog.urth.org
https://github.com/autarch

On Mon, Jul 10, 2017 at 1:02 PM, Binarus <lists@binarus.de> wrote:

> Dear experts,
>
> a few days ago I have got great help from this list, so I hope I may ask
> another (probably stupid) question (I am now having the opposite problem
> than back then):
>
> Using DateTime, is it possible to tell in advance if a certain date-time
> which is given in a certain locale will be ambiguous due to switching
> from DST to standard time?
>
> Example (taken from DateTime's documentation):
>
>   my $dt = DateTime->new(
>       year      => 2003,
>       month     => 10,
>       day       => 26,
>       hour      => 1,
>       minute    => 30,
>       second    => 0,
>       time_zone => 'America/Chicago',
>   );
>
> Now $dt is ambiguous: The clock has been turned back to 01:00:00 h at
> 02:00:00 that night, so the time 01:30:00 h has occurred twice.
>
> Even after thinking many hours about it, I haven't found a reasonable
> general method to determine if an arbitrary date-time is ambiguous in
> the sense above.
>
> Possibly, I could subtract different time spans from the date-time in
> question and check if the result is the expected one and use that to
> find out if it is ambiguous, but this would cost much CPU time.
>
> So I would like to ask if somebody knows a general, reasonable method
> for solving that problem, given the following conditions:
>
> 1) We don't know the time span the clock is turned back when switching
> from DST to standard time. It might be one hour in most time zones /
> countries, but after all, some weird person could decide that it is 18
> minutes and 13 seconds or 5 hours, 53 minutes and 42 seconds.
>
> 2) We don't know whether the point in time when the switch occurs is
> exactly at an hour's end / begin. Again, some weird person could decide
> that the switch happens at 08:48:27 am.
>
> I know that there are not many persons on the world that are *that*
> weird, but on the other hand, I don't want to implement an algorithm
> which uses assumptions which are not safe.
>
> As a last resort, there is at least one other module (AFAIK) which I
> could use to extract the daylight saving switching times and time spans
> from the time zone database, and I could use that information to solve
> my problem. But this would probably mean to reinvent the wheel, so I'd
> like to avoid it.
>
> Thank you very much in advance,
>
> Binarus
>

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