From the Perl Cookbook

use Date::Calc qw(Day_of_Week Week_Number Day_of_Year);
# you have $year, $month, and $day
# $day is day of month by definition
$wday = Day_of_Week($year, $month, $day);
$wnum = Week_Number($year, $month, $day);
$dnum = Day_of_Year($year, $month, $day);
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Update I read that backwords expected numeric to name. So how would we reverse it hmmmm.... I would go with shockme because depending on how deep you have to go back your function mishandles leap years.