Translate getdate.y into C for portability. Make the get_date()
function easier to test as well:
* Have it accept a time_t "now" to use as a reference so that test
code can verify relative time specifications against known starting
points.
* Set up default date after parsing the string so that we
can use the specified timezone (if any) instead of the local
default. Otherwise, local DST makes it almost impossible to
reliably test time specifications such as "sunday UTC"
