for (my $i = 0; $i < 60; $i++) {

    my $offset = getOffsetToNearestInterval($i,15);
    print "$i: " . ($i + $offset) . ' (' . $offset . ')' . "\n";

}

sub getOffsetToNearestInterval {

    my $number = shift;
    my $interval = shift;

    my $nearest_interval = ( $interval * int( ($number + ($interval/2) ) / $interval) );
    my $offset_to_interval = $nearest_interval - $number;

    return $offset_to_interval;
}