The week number can be described by counting the Thursdays: week 12 co
+ntains the 12th Thursday of the year.

The ISO week-numbering year starts at the first day (Monday) of week 0
+1 and ends at the Sunday before the new ISO year (hence without overl
+ap or gap). It consists of 52 or 53 full weeks. The first ISO week of
+ a year may have up to three days that are actually in the Gregorian 
+calendar year that is ending; if three, they are Monday, Tuesday and 
+Wednesday. Similarly, the last ISO week of a year may have up to thre
+e days that are actually in the Gregorian calendar year that is start
+ing; if three, they are Friday, Saturday, and Sunday. The Thursday of
+ each ISO week is always in the Gregorian calendar year denoted by th
+e ISO week-numbering year.

Examples:

Monday 29 December 2008 is written "2009-W01-1"
Sunday 3 January 2010 is written "2009-W53-7"
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#!/usr/bin/perl
use strict;
use warnings;
use Date::Manip;
use feature 'say';

my $datestr = ParseDate("01/01/2016");
my $secs = UnixDate($datestr,'%s');
say ParseDateString("epoch $secs");
say UnixDate($datestr,"%J");

__END__

$ perl test.pl
2016010100:00:00
2015-W53-5
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