getting minutes difference between two Time::Piece objects - perl

Question

I have the code:
use Time::Piece;
use Time::Seconds;
my $timespan = $latest_time - $timestamp;
print $latest_time . "\n";
print $timestamp . "\n";
print $timespan->minutes;
where $latest_time = Time::Piece->new; and $timestamp = Time::Piece->strptime();
and I get the results:
Thu Mar 27 09:40:19 2014
Thu Mar 27 09:40:00 2014
-479.683333333333
What went wrong? there should be 0 minutes for $timespan, correct? Where is -479 coming from?

Answer 1

Reproducing the "bug"
This issue arises because strptime defaults to UTC instead of to the local timezone. This can be demonstrated in the following code which takes a current time, prints it out, then reparses it and shows the difference:
use strict;
use warnings;
use Time::Piece;
my $now = Time::Piece->new();
print $now->strftime(), "\n";
my $fmt = "%Y-%m-%d %H:%M:%S";
my $nowstr = $now->strftime($fmt);
my $parsed = Time::Piece->strptime("$nowstr", $fmt);
print "($nowstr)\n";
print $parsed->strftime(), "\n";
my $diff = $now - $parsed;
print $diff->hours, " hours difference\n";
Outputs:
Wed, 26 Mar 2014 21:42:08 Pacific Daylight Time
(2014-03-26 21:42:08)
Wed, 26 Mar 2014 21:42:08 UTC
7 hours difference
One hackish solution - getting parsed times to read as local
Now, in hacking around, I've discovered one potential hack for this on my strawberry perl system. It's by calling strptime like this: $now->strptime.
my $nowstr = "2014-03-26 21:51:00"; #$now->strftime($fmt);
my $parsed = $now->strptime("$nowstr", $fmt); #Time::Piece->strptime("$nowstr", $fmt);
print "($nowstr)\n";
print $parsed->strftime(), "\n";
my $diff = $now - $parsed;
print $diff->hours, " hours difference\n";
To confirm that strptime was actually using the time I set it, I gave it one that was 6 minutes before the current time. The output is as follows:
Wed, 26 Mar 2014 21:57:00 Pacific Daylight Time
(2014-03-26 21:51:00)
Wed, 26 Mar 2014 21:51:00 Pacific Standard Time
0.1 hours difference
The parsed time will inherit the c_islocal value from $now. $now just needs to be initialized with either localtime or ->new() and not gmtime of course.
As you can see one claims DST while the other does not, but date math is still done correctly. I was able to figure out this hack by looking at the source for strptime, _mktime, and new.
Hopefully, at the very least my code to reproduce the error will be helpful to someone with more experience with Time::Piece, and I'd love a better solution.

Answer 2

When I use strftime("%H:%M:%S %Z") for both $latest_time and $timestamp they both show the same timezone, but the $latest_time - $timestamp operation shows that there is difference between the timezones as tobyink pointed out. This might be a bug in Time::Piece module.
So it seems that Time::Piece->new; gets the current machine time including the timezone.
So either I fix the timezone in Time::Piece->new or include a timezone value when I use Time::Piece->strptime(); to fix the timezone problem. Thanks for the info, tobyink.