Fix more date bugs.

Fix more date bugs.

Correctly and portably check for non-normal dates, and explicitly show
the "before" and "after" dates (in the format of the user's choosing).
Clear the struct tm in date_main rather than parse_default because on
one path the struct tm is actually initialized. Explicitly clear the
tm_sec field in parse_default because -- experiment shows -- that
should not be preserved. Only do the "what does this 2-digit year
mean?" dance if we actually parsed a 2-digit year. Show the right
string in the error message if strptime fails.

Also add more tests, and use UTC in the tests to avoid flakiness.