[Datetime-SIG] Another round on error-checking

[Chris Barker - NOAA Federal]

> It's unreasonable to ask people to settle for arithmetic at best 10x
> slower just to get correct timezone conversions

I'm not sure. As has been pointed out, best practice is to use UTC or
naive time anyway.

So if the casual user wants to compute how long s/he slept last night,
it can be slow. It's easier to document "computations are much faster
in UTC" than to document all the surprising inconsistencies.

And as for original intent -- my understanding of the entire
architecture was designed NOT to be about fast arithmetic. If you want
that, use tics or numpy.datetime64.

And intentional or not, "classic" arithmetic may be easy to implement
and fast, but it is hard to explain, surprising, and not very useful.

-Chris