Not just "considered", the Date::Manip documentation has an entire "should I use Date::Manip" section in which the modules author explains at great length that Date::Manip is by far the most flexible date/time module out there, but also the slowest (in large part because of that extreme flexibility).
Well, that section more or less predates the DateTime project, to the best of my knowledge.
Date::Manip is quite slow, but the DateTime project has never made speed anywhere near as high a priority as correctness, completeness, and having a sane API.
The original poster didn't make it entirely clear what he wants to do, but the Perl Advent calendar article mentioned in another post is a decent pointer for using DateTime is a good start.