jdtoronto has asked for the wisdom of the Perl Monks concerning the following question:

Esteemed monks,

I have an application where I am usinf Time::Piece to parse times and do some calculations based on them. But today I discovered a wierdness which leads me to believe that the module does not understand Time at all!

If I take a localtime and create an object $t, then I take a string and parse it using either the 

Time::Piece->from_mysql_datetime()

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method or the 
Time::Piece->strptime()

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method, then next I subtract one from the other. I always get a difference which is 5 hours too great! The following code will demonstrate the problem (Well I hope it will on your machine!) I just need a sanity check form someone before I send this to the module author.

The code 

#!/usr/local/bin/perl

use strict;
use warnings;

use Time::Piece::MySQL;
use Time::Seconds;
use Data::Dumper;

my $time = localtime;
my $future_time = $time + ONE_HOUR;

my $diff = $future_time - $time;

print "Local time:  " . $time . "\n";
print "Future time: " . $future_time . "\n";
print "Difference:  " . $diff->hours . "hours \n";

print Dumper( $time, $future_time, $diff);

my $test_time = Time::Piece->from_mysql_datetime( '2004-03-05 20:30:00
+' );
print Dumper $test_time;
print "Test time: " . $test_time . "\n";
$diff = $test_time - $time;
print $diff->minutes . " minutes\n";

$test_time = Time::Piece->strptime( '2004-03-05 20:30:00', '%Y-%m-%d %
+H:%M:%S' );
print Dumper $test_time;
print "Test time: " . $test_time . "\n";
$diff = $test_time - $time;
print $diff->minutes . " minutes\n";

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My sample output: 
[root@posiedon3 johnday]# perl timepiece.pl
Local time:  Fri Mar  5 20:44:17 2004
Future time: Fri Mar  5 21:44:17 2004
Difference:  1hours 
$VAR1 = bless( [
                 17,
                 44,
                 20,
                 5,
                 2,
                 104,
                 5,
                 64,
                 0,
                 1078537457,
                 1
               ], 'Time::Piece' );
$VAR2 = bless( [
                 17,
                 44,
                 21,
                 5,
                 2,
                 104,
                 5,
                 64,
                 0,
                 1078541057,
                 1
               ], 'Time::Piece' );
$VAR3 = bless( do{\(my $o = 3600)}, 'Time::Seconds' );
$VAR1 = bless( [
                 0,
                 30,
                 20,
                 5,
                 2,
                 104,
                 5,
                 64,
                 0,
                 undef,
                 0,
                 0
               ], 'Time::Piece' );
Test time: Fri Mar  5 20:30:00 2004
-314.283333333333 minutes
$VAR1 = bless( [
                 0,
                 30,
                 20,
                 5,
                 2,
                 104,
                 5,
                 64,
                 0,
                 undef,
                 0,
                 0
               ], 'Time::Piece' );
Test time: Fri Mar  5 20:30:00 2004
-314.283333333333 minutes

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Could someone please check what I have done? Many thanks!

jdtoronto

Replies are listed 'Best First'.
Re: Time::Piece wierdness - sanity check please!
by tachyon (Chancellor) on Mar 06, 2004 at 02:00 UTC

    You are trying to use the string returned by a scalar context call to locatime() as the integer unix epoch time you really mean. You can't add ONE_HOUR to 'Sat Mar 6 01:55:26 2004' and get what you want! 

    my $time = localtime;  # you mean $time = time();
print $time, $/;
$time = time();
print $time, $/;
__DATA__
Sat Mar  6 01:55:26 2004
1078538126

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    cheers

    tachyon

[reply]
[d/l]
      Time::Piece over-rides localtime and gmtime, ONE_HOUR is a constant exported by Time::Seconds.

      $t = localtime - returns an object which, when stringified, behaves the same as the original localtime.

      The time arithmetic is not the problem, the problem is when you parse a datetime you get a value which is being treated as though it were GMT. Hence the difference values being calculated later which are 5 hours greater than the actual difference between the times.

      jdtoronto

[reply]
        Yes, I get equivalent results -- I seem to be in the same time zone as you, and get a similar 5-hour "padding" of the difference between "localtime" and a given time string passed to either "from_mysql_datetime" or "strptime". Regarding this latter method, the Time::Piece man page says "see the [unix] strptime man page", and when I looked at that (on MacOSX BSD), it said "The resulting values will be relative to the local time zone." But it looks like Time::Piece is not treating it as such -- instead, it's treating the values as relative to GMT -- and so is behaving badly (or at least counter to apparent intention, which is bad).
[reply]

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