This Input Handler verifies that it is dealing with a reasonable date.
Reasonably means anything that Date::Manip thinks is sensible, so you
could use any of (for example): "December 12, 2001" "12th December, 2001"
"2001-12-12" "next Tuesday" "third Wednesday in March"

See Date::Manip for much more information on what date formats are
acceptable.

The resulting date will be a Date::Simple object. Date::Simple for more
information on this.