As SuicideJunkie pointed out "0150" is not the same as "1350". But here is some pared down code to convert "HH:MI" timestamps to "epoch":

use Time::Local;

my $t1   = "1245";
my $t2   = "1350";

my $beg  = ConvHHMI($t1);
my $end  = ConvHHMI($t2);

my $diff = $end - $beg;
my $t3   = $diff / 60;

print "Begin=$t1 ($beg), End=$t2 ($end), Diff=$t3 ($diff)\n";

sub ConvHHMI {
    my  $w_date      = shift @_;
    my ($sec,$min,$hour,$mday,$mon,$year,$misc) = (0,0,0,1,0,0,0);
    ($hour, $min)    = $w_date =~ /^(\d\d)(\d\d)$/;
    return timelocal($sec,$min,$hour,$mday,$mon,$year);
}
[download]

Results:

Begin=1245 (946759500), End=1350 (946763400), Diff=65 (3900)