I've worked with both Date::Manip and Date::Calc and would describe the difference like this: Date::Calc is great for working with structured dates. So if you are getting data from web forms where you can control the input format, sticking it in database, and printing it back out, then Date::Calc is a great choice.

Date::Manip has the strength of being able to parse all manner of english descriptions of dates and "date math" into a real date format and spit back out lots of ways. The project we used was on was noticeably slower as a CGI script, which was one of the reasons we explored Date::Calc.

I use Date::Calc as my first choice date parsing module and I'm very happy with it.

-mark