Someone offline suggested this to me. Ok, fair enough you don't reinvent the wheel unless you have too or for educational purposes but leaving that aside for the moment is messing with dates one of those things like parsing HTML that you're just better off not trying to do from scratch ?

#!/usr/bin/perl -w

use strict;

use Time::Local qw( timelocal );

my $year = 2001;
my $day = 200;

my $year_sec = timelocal(0, 0, 0, 1, 0, $year-1900);
my $new_sec = $year_sec + ($day-1) * 60 * 60 *24;

print localtime($new_sec)."\n";

my ($theday, $month) = (localtime($new_sec))[3,4];
$month++;

print "day = $theday, month = $month\n";
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