[PATH] Correct negative/zero year in to_date/to_timestamp

Hello, Hackers!

I'm writing another patch and while I was trying to cover corner cases
I found that to_date and to_timestamp work wrong if year in input
value is zero or negative:

postgres=# SELECT
postgres-#      y || '-06-01' as src
postgres-#     ,CASE WHEN y>0 THEN ('00'||y||'-06-01') WHEN y<0 THEN
('00'||(-y)||'-06-01 BC') END::date
postgres-#     ,to_date(y || '-06-01', 'YYYY-MM-DD')
postgres-#     ,to_timestamp(y || '-06-01', 'YYYY-MM-DD')
postgres-# FROM (VALUES(2),(1),(0),(-1),(-2))t(y);
   src    |     date      |    to_date    |       to_timestamp
----------+---------------+---------------+---------------------------
 2-06-01  | 0002-06-01    | 0002-06-01    | 0002-06-01 00:00:00+00
 1-06-01  | 0001-06-01    | 0001-06-01    | 0001-06-01 00:00:00+00
 0-06-01  |               | 0001-06-01 BC | 0001-06-01 00:00:00+00 BC
 -1-06-01 | 0001-06-01 BC | 0002-06-01 BC | 0002-06-01 00:00:00+00 BC
 -2-06-01 | 0002-06-01 BC | 0003-06-01 BC | 0003-06-01 00:00:00+00 BC
(5 rows)

Zero year (and century) is accepted and negative years differs by 1
from what they should be.


I've written a patch fixes that. With it results are correct:
postgres=# SELECT
postgres-#      y || '-06-01' as src
postgres-#     ,CASE WHEN y>0 THEN ('00'||y||'-06-01') WHEN y<0 THEN
('00'||(-y)||'-06-01 BC') END::date
postgres-#     ,to_date(y || '-06-01', 'YYYY-MM-DD')
postgres-#     ,to_timestamp(y || '-06-01', 'YYYY-MM-DD')
postgres-# FROM (VALUES(2),(1),(-1),(-2))t(y);
   src    |     date      |    to_date    |       to_timestamp
----------+---------------+---------------+---------------------------
 2-06-01  | 0002-06-01    | 0002-06-01    | 0002-06-01 00:00:00+00
 1-06-01  | 0001-06-01    | 0001-06-01    | 0001-06-01 00:00:00+00
 -1-06-01 | 0001-06-01 BC | 0001-06-01 BC | 0001-06-01 00:00:00+00 BC
 -2-06-01 | 0002-06-01 BC | 0002-06-01 BC | 0002-06-01 00:00:00+00 BC
(4 rows)


When year "0" is given, it raises an ERROR:
postgres=# SELECT to_timestamp('0000*01*01', 'YYYY*MM*DD');
ERROR:  invalid input string for "YYYY"
DETAIL:  Year cannot be 0.


Also I change behavior for era indicator when negatives century or
year are given. In such case era indicator is ignored (for me it is
obvious signs should be OR-ed):
postgres=# SELECT to_timestamp('-0010*01*01 BC', 'YYYY*MM*DD BC')
postgres-#       ,to_timestamp(' 0010*01*01 BC', 'YYYY*MM*DD BC');
       to_timestamp        |       to_timestamp
---------------------------+---------------------------
 0010-01-01 00:00:00+00 BC | 0010-01-01 00:00:00+00 BC
(1 row)


Testings, complains, advice, comment improvements are very appreciated.

--
Best regards,
Vitaly Burovoy