parse-datetime: allow calculations to yield -1 Problem reported by Jeremy Cantrell <https://bugs.gnu.org/50115>. * lib/parse-datetime.y (parse_datetime_body): When calling mktime, use an unmodifed and negative tm_wday or tm_yday to detect an error, as a (time_t) -1 return value is valid on most hosts. * tests/test-parse-datetime.c (main): Add a test for the bug.