Take a look at Date::Manip module on CPAN. It can do this and a whole load more.

For your specific problem, the following code:

use Date::Manip;

print UnixDate( ParseDate( "last business day of september 2002"), '%F
+' );
[download]

gives: Monday, September 30, 2002

BTW: It would be easier for thise coming along after you to find the answers already given to your problem if you would make your titles a little more specific to the question you are asking.

Suggestion: "A function to find last business day of the month." - maybe too wordy, but you get the idea :^)