Two examples. (didactical purpose)

#!/usr/bin/perl 
use strict;
use warnings;
use v5.10.0;
use Time::Piece;
use Time::Seconds;
use DateTime;


my $dt = DateTime->now( time_zone => 'floating' );

my $next = { 
    mon => ( $dt->mday >= 21 ? ( $dt->mon + 1 ) % 12 : $dt->mon ),
    year =>
      ( ( $dt->mon == 12 and $dt->mday >= 21 ) ? $dt->year + 1 : $dt->
+year ),
    day => 21, 
    hms => '12:00:00'
};

$next->{'dt'} = DateTime->new(
    year  => $next->{'year'},
    month => $next->{'mon'},
    day   => $next->{'day'},
    hour  => substr( $next->{'hms'}, 0, 2 ) 
);

my $t = localtime;

print Time::Seconds->new(
    Time::Piece->strptime(
        sprintf( "%s-%s-%sT%s",
            $next->{'year'}, $next->{'mon'}, $next->{'day'}, $next->{'
+hms'} ),
        "%Y-%m-%dT%T"
      ) - $t
)->seconds, "\n";

print $next->{'dt'}->subtract_datetime_absolute($dt)->seconds, "\n";

# Output :
#  1308641
#  1308641
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