5. Reset $ENV{TZ} to its original value so it doesn't muck up any other date calculations elsewhere in the code

You could of course localize %ENV:

{
    local $\ = "\n";
    print scalar localtime;
    {
        local %ENV;
        $ENV{TZ} = 'GMT';
        print scalar localtime;
    }
    print scalar localtime;
}
[download]

Wed May 29 23:29:08 2002
Wed May 29 21:29:08 2002
Wed May 29 23:29:08 2002
[download]

Update/PS: -- for frontpaging your own node. Don't do that.

- Yes, I reinvent wheels.
- Spam: Visit eurotraQ.

You don't have to localize all of %ENV:

local $ENV{TZ} = 'GMT';
[download]

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print

You don't have to localize all of %ENV: local $ENV{TZ} = 'GMT';

Please test before posting. Localizing a single %ENV value will not work as expected.

{
    local $\ = "\n";
    print scalar localtime;
    {
        local $ENV{TZ} = 'GMT';
        print scalar localtime;
    }
    print scalar localtime, ", which is wrong.";
}
[download]

yields:

Thu May 30 23:19:56 2002
Thu May 30 21:19:56 2002
Thu May 30 21:19:56 2002, which is wrong.
[download]

Update: environment: Debian GNU/Linux sid x86, Perl 5.6.1

- Yes, I reinvent wheels.
- Spam: Visit eurotraQ.

Suppose I give you the date '27 October 2002, 2:30', and the timezone is CET. Is that in daylight savings time or not? The answer is that *both* are possible. There will be a 27 October 2002, 2:30 in DST, followed one hour later by a 27 October 2002, 2:30 not in DST.

Of course it's possible that given a date (and time), and the timezone to calculate which date/times have to be DST, which ones cannot be DST, which ones can be both (typically just one hour/year), and which date/time combinations are impossible (typically one hour/year as well). After all, that's what your computer does too when displaying time ;-) But the rules differ from timezone to timezone.

Note that in your particular example, with TZ = MST7MDT, it's easy, as then the timezone name itself indicates daylight savings time is active.

I don't understand your steps to determine DST though. With step 1, you convert the date to epoch seconds - but you already need to know the timezone and whether it's DST or not for that. And why are you in step 3 fiddling with the epoch?

Finally, did you study Time::Local and see how that tackles the problem?

Abigail

Late reply, but:

This is absolutely impossible without knowing local rules. Because there can be different rules within a single timezone. Example: while Arizona in its entirety belong to the Mountain time zone, most (but not all) of it does NOT observe Daylight Savings Time. Other states in this same timezone do. There are a number of other similar situations in the world.

Your best bet is to query some service for example geonames. Then you can have accurate data.

If the goal is to have two different systems in different timezones have a normalized time, why not use gmtime() instead of localtime()? it gives you the same value regardless of the local...

/\/\averick
OmG! They killed tilly! You *bleep*!!

Thanks for your reply!

The reason is that I'm doing timezone conversion. The code is supposed to be high-availability ... it is very bad if times get snookered even a couple times a year.

I'm using Date::Manip to perform the conversion. Date::Manip understands MST and MDT, but it does not understand something like MST7MDT. For example,

#!/usr/bin/perl
use strict;
use Date::Manip;
my $date = &ParseDate("2002-05-29 08:00:00");
print "The unconverted date is [".&UnixDate($date,"%Y-%m-%d %H:%M:%S")
+."]\n";
my $from = 'MDT';
my $to = 'GMT';
my $converted = &Date_ConvTZ($date,$from,$to);
print "The conversion from [$from] resulted in [".&UnixDate($converted
+,"%Y-%m-%d %H:%M:%S")."]\n";
$from = 'MST7MDT';
$converted = &Date_ConvTZ($date,$from,$to);
print "The conversion from [$from] resulted in [".&UnixDate($converted
+,"%Y-%m-%d %H:%M:%S")."]\n";
[download]

gives

The unconverted date is [2002-05-29 08:00:00]
The conversion from [MDT] resulted in [2002-05-29 14:00:00]
The conversion from [MST7MDT] resulted in [2002-05-29 08:00:00]
[download]

So I need to know whether a given date is in Mountain Daylight Time or Mountain Standard Time.

Edit by tye to change PRE tags to CODE tags

So essentially the problem is that Date::Manip doesn't understand MST7MDT and gives you GMT? Maybe you could look at the internals of Date::Manip and add that timezone? Perhaps a different CPAN module would do the trick...Date::Handler looks promising..

HTH

/\/\averick
OmG! They killed tilly! You *bleep*!!

I'm fairly certain that Time::ParseDate correctly handles the AAAXBBB timezone format.

Update: Whoops, I'm fairly certain I'm wrong.

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print

Not that I particularly worry about XP, but I would be appreciative if someone could puhleeze clue me in to my why it seems to be receiving a negative response? Did I ask a stupid question? Was the question offensive? Not enough detail? Too much detail? Purely random voting?

I'm a bit confused at this point. I'd like to ask questions in such a way as to positively contribute to the community, so any feedback would be appreciated.

Update: Just saw Juerd's "--" comment. Didn't know that was a no-no. Thanks, Juerd :-)