Using strptime to parse timestamp relative to the local time - perl

Question

I am trying to do some date calculation relative to the current local time.
For example:
use feature qw(say);
use strict;
use warnings;
use Time::Piece;
my $fmt = '%Y-%m-%d_%H:%M:%S';
my $timestamp = "2015-04-12_11:07:27";
# This gives incorrect $t1 relative to localtime
my $t1 = Time::Piece->strptime( $timestamp, $fmt );
my $t2 = localtime;
say "Local time: " . localtime;
say "Local time epoch: " . time;
say $t1->epoch();
say $t2->epoch();
my $timestamp1 = $t1->strftime( $fmt );
my $timestamp2 = $t2->strftime( $fmt );
say $timestamp1;
say $timestamp2;
my $delta = $t2 - $t1;
say $delta;
A sample output:
Local time: Sun Apr 12 12:21:49 2015
Local time epoch: 1428834109
1428836847
1428834109
2015-04-12_11:07:27
2015-04-12_12:21:49
-2738
Which clearly gives the a wrong time difference of -2738. ( It should be a positive number)

Answer 1

If the date-time you parse has no time zone information, it's assumed to be UTC. You can see this by adding the following two lines in your script:
say "tzo1 = ",$t1->tzoffset;
say "tzo2 = ",$t2->tzoffset;
In Paris, the above outputs the following:
tzo1 = 0
tzo2 = 7200
You can override the default to be the local time zone by using the undocumented feature of using localtime instead of Time::Piece as the invocant.
$ perl -MTime::Piece -E'
say Time::Piece->strptime("2015-04-12_11:07:27", "%Y-%m-%d_%H:%M:%S")->tzoffset;
say localtime ->strptime("2015-04-12_11:07:27", "%Y-%m-%d_%H:%M:%S")->tzoffset;
'
0
7200
Doing that minor change gives the answer you were expecting.
$ perl -MTime::Piece -E'
say localtime->strptime("2015-04-12_11:07:27", "%Y-%m-%d_%H:%M:%S") - localtime;
'
5524

Answer 2

I think this can be done using Date::Time :
use feature qw(say);
use strict;
use warnings;
use DateTime;
use DateTime::Format::Strptime;
use DateTime::Duration;
my $strp = DateTime::Format::Strptime->new(
pattern => '%Y-%m-%d_%H:%M:%S',
time_zone => 'local',
);
my $timestamp = "2015-04-12_11:07:27";
my $dt1 = $strp->parse_datetime( $timestamp );
my $dt2 = DateTime->now();
say $dt2->subtract_datetime_absolute( $dt1 )->seconds();