Thanks. In the patch, "without a sign" only holds for years after
9999: dropping the minus from -0001-01-02 gives 0001-01-02, which is
1 AD, not 2 BC. Perhaps:
(ISO 8601 writes years after 9999 with a leading plus sign, and
years before 1 AD from a year zero, so 0000 is 1 BC and -0001 is
2 BC. PostgreSQL accepts neither; write 10000-01-02 and
0002-01-02 BC instead.)