why are timegm and timelocal returning the same epoch seconds values

[kehansen]

I have a script to post-process a data file that contains multiple timestamps in GMT. This script is to convert the timestamps from GMT to specific timezone equivalent. My logic is to convert the timestamp to epoch seconds, determine the seconds difference between GMT and local time, subtract this from the epoch seconds, and convert this back to a local timestamp. The problem I am seeing is that in trying to get the difference between GMT & local time the returned epoch second are the same. Here is my sub-routine to get the difference.

sub getGMDelta
  {
    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
                                                localtime(time);
    $ltime = timelocal($sec,$min,$hour,$mday,$mon,$year);
    $gtime = timegm($sec,$min,$hour,$mday,$mon,$year);
    $rv = $gtime - $ltime;
    return($rv);
}

[Laurent_R]

The number of seconds since the epoch is independent of the time zone. What depends on the time zone is the way this number of se conds is interpreted.

Maybe this session under ste Perl debugger will clarify what happens:

  DB<1>  print scalar gmtime time
Tue May 14 22:27:51 2013
  DB<2> print scalar localtime time
Wed May 15 00:27:59 2013
  DB<3>  print time
1368570496
  DB<4> @c =  gmtime 1368570496

  DB<5> x @c
0  16
1  28
2  22
3  14
4  4
5  113
6  2
7  133
8  0
  DB<6> @d =  localtime 1368570496

  DB<7> x @d
0  16
1  28
2  0
3  15
4  4
5  113
6  3
7  134
8  1
  DB<8>

My apologies if I did not really answer to your question, I am not entirely sure what your problem is.

[karlgoethebier]

Your sub is OK. For me it returns 7200 - GMT+2.

For calculating time delta you could take a look at Date::Calc and/or DateTime.

Update:

Perhaps calculating the time delta from the timestamps in your file(s), not from the current time? Or do i miss something?

Update2:

I'm still struggling a bit with this.

As far as i understood your timezone is GMT.

You have a file with some timestamps you think they are in another timezone, right?

And you want to know the difference between this timestamps and GMT, right?

So IMHO your sub needs the timestamps from your file as params...

Update3:

«why are timegm and timelocal returning the same epoch seconds values»

Because your box is GMT?

Update4:

Just as a little addendum (assuming you are using bash):

Karls-Mac-mini:monks karl$ date
Di 14 Mai 2013 23:22:21 CEST
Karls-Mac-mini:monks karl$ export TZ=UTC
Karls-Mac-mini:monks karl$ date
Di 14 Mai 2013 21:22:47 UTC
Karls-Mac-mini:monks karl$ unset TZ
Karls-Mac-mini:monks karl$ date
Di 14 Mai 2013 23:23:11 CEST

Update5:

OK, this is the last one, i promise...

#!/usr/bin/env perl 
# delta.pl
use strict;
use warnings                                                          
+                 

use Time::Local; 

print getGMDelta() . qq(\n);

sub getGMDelta
{
    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
        localtime(time);
    $ltime = timelocal($sec,$min,$hour,$mday,$mon,$year);
    $gtime = timegm($sec,$min,$hour,$mday,$mon,$year);
    $rv = $gtime - $ltime;
    return($rv);
}

__END__

Karls-Mac-mini:monks karl$ export TZ=UTC
Karls-Mac-mini:monks karl$ ./delta.pl
0
Karls-Mac-mini:monks karl$ unset TZ
Karls-Mac-mini:monks karl$ ./delta.pl
7200

Please see also:

• Timezone
• UTC
• Greenwich Mean Time
• Central European Time

Regards, Karl

[vsespb]

For me this code works. Why do you think your system's timezone is not GMT+0 ?

[Anonymous Monk]

For me this code works. Why do you think your system's timezone is not GMT+0 ?

Since the code is incomplete (  Undefined subroutine &main::timelocal called ) how is it possible that this code works for you?

[vsespb]

I googled for "perl timegm" and found that I need to

use Time::Local;

Perhaps OP use different module and that's the reason of problem?

[Not_a_Number]

Works for me too (with the addition of use Time::Local; at the top).

However, I would simply the sub somewhat:

sub getGMDelta {
  my @now = localtime;
  return timegm( @now ) - timelocal( @now );
}

[kehansen]

Thank you for the responses. Here is more information. I do have "use Time::Local" at the top of my main script. My host is running Solaris 10 and the timezone is set for "US/Pacific". The file I am processing contains multiple(24) timestamps that are in GMT time and we have a customer who wants these times converted to local time.

[vsespb]

1. Try extract this subroutine from you script and run it outside of your script.

2. Is your TZ variable set?

3. Try run with env TZ=US/Pacific

4. Try run system tools (like 'date) which report time to make sure they recognize timezone correctly

[karlgoethebier]

Update2:

use DateTime;
use DateTime::TimeZone;

my $dt = DateTime->new(
      year       => 2013,
      month      => 4,
      day        => 15,
      hour       => 12,
      minute     => 45,
      second     => 47,
      time_zone  => 'Europe/Berlin',
  );

my $tz = DateTime::TimeZone->new( name => 'US/Pacific' );

my $offset = $tz->offset_for_datetime($dt);

print qq($offset\n);

print DateTime::TimeZone->offset_as_string( $offset ) . qq(\n);

print qq($dt->hour\n); 

$dt->set_time_zone( 'US/Pacific' );

print qq($dt->hour\n);

Regards, Karl