[development] no date / time types in schema API

[Chris Johnson]

Note that Drupal 6 uses 32-bit signed integers in the database (MySQL)
for UNIX timestamps, and thus has a range of -2147483648 to 2147483647
seconds, or Fri, 13 Dec 1901 to Tue, 19 Jan 2038.  And that's only if
you use PHP to do the Unix time to date conversion.  MySQL stupidly
does not handle the full range, and will return NULL for values near
the end points (how near, I don't know -- I didn't test more than a
few values).

So, 64-bit timestamps might allow us to represent all useful dates
ever, but we are not using 64-bit timestamps and it's unlikely we will
require such within the next year.