From perldoc Date::Manip...

** Note that the formats "sunday week 22" and "22nd sunday" give very different bahaviors. "sunday week 22" returns the sunday of the 22nd week of the year based on how week 1 is defined. ISO 8601 defines week one to contain Jan 4, so "sunday week 1" might be the first or second sunday of the current year, or the last sunday of the previous year. "22nd sunday" gives the actual 22nd time sunday occurs in a given year, regardless of the definition of a week.

-- Randal L. Schwartz, Perl hacker