The functionality provided by DateTime absolutely should be in core.
Eek, no! The DT stuff needs to keep up with leap second and time zone change announcements, so it needs its own release schedule. We'd have to figure out a way to release data packages separately from what's in the core.
Of course, in theory, I can see why this should be in the core, just because it's the kind of low-level data type that you want for programming.
But given how easy CPAN is to use, is it really a big barrier? I think the big problem, as usual, is that there's lots of similar stuff to wade through. Fortunately, DateTime seems to have enough of a following these days that every time there's a datetime programming question, at least one respondent says "use DateTime", so the word is out.