I prefer Time::Piece over Date::Manip. There is also the DateTime suite (not Date::Time) ... both of these will allow you to add and subtract dates fairly easily. If you still can't get Date::Manip to install, then perhaps you can try one of these alternatives until you or someone else figures out why the install didn't work.

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)