If you read the docs for Date::Manip, you'll see that even the author admits that it's a very large and slow module. The only time that I'd consider using it would be to parse date strings. For most other date calculations the standard Perl functions are much more efficient (like chromatic's Week number example above.

--
<http://www.dave.org.uk>

European Perl Conference - Sept 22/24 2000, ICA, London
<http://www.yapc.org/Europe/>