my %month_nums;
for (my $i = 0; $i <= $#months; $i++) {
   $month_nums{$month[$i]} = $i;
}

# This creates a hash of 'Jan' => 0, 'Feb' => 1, etc.

sub change_month_to_num {
    my $month = shift;
    return $month_nums{$month} + 1;
}