Actually, DateTime is already becoming more of a standard than Date::Manip, because it is designed to play well with others. So we see other, non-datetime module authors supporting it. For example, in SPOPS, Chris Winters now support DateTime objects for setting datetime values in a database. He also supports Time::Piece & Class::Date, but it defaults to DateTime.

And other datetime module authors are starting to look to DateTime as a standard as well. Matt Sergeant has a working beta of Time::Piece 2.0x, which will simply be a DateTime subclass. He has said that he hopes to see users of Time::Piece eventually move to DateTime itself. Similarly, Graham Barr asked me not long ago when DateTime would replace his TimeDate distro.

People like DateTime because it not only does everything they need, but it has a design, and a plan for the future, and it plays really nice with everyone else. Date::Manip does a lot of stuff, but the API is a mess, and it doesn't integrate well with any other datetime modules. Similarly, Date::Calc also does a lot of stuff, but it doesn't play well with others at all.