BUG #16216: the result of to_date function with negative year number not same as BC year number

The following bug has been logged on the website:

Bug reference: 16216
Logged by: zhongxuan sun
Email address: szx9231(at)gmail(dot)com
PostgreSQL version: 12.0
Operating system: centos7
Description:

example:
postgres=# select to_date('-120', 'yyyy');
to_date
---------------
0121-01-01 BC
(1 row)

postgres=# select to_date('120bc', 'yyyybc');
to_date
---------------
0120-01-01 BC
(1 row)

I think -120 means 120bc, however, the output is 121bc not 120bc.

Thanks~