Here is a solution that uses a different Package, Date::Manip.

use strict;
use Date::Manip;

my $arg = shift || "";

my $start;
my $end;

if (my ($day, $hours) = ($arg =~ m!^(\w{3})/(\d+)!)) {
    my $err;
    my $start = ParseDate($day);

    die "Bad day $day" unless $start;

    print UnixDate($start, "%u"), "\n";
    $end = DateCalc($start,"+ $hours hours", \$err);
    print UnixDate($end, "%u"), "\n";
} else {
    die( "Error: use an argument of dow/hours (e.g. Sat/36)" );
}
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