BUG #6231: weird to_timestamp behaviour with out of range values

The following bug has been logged online:

Bug reference: 6231
Logged by: Henk Enting
Email address: h(dot)d(dot)enting(at)mgrid(dot)net
PostgreSQL version: 9.1.1
Operating system: linux x86_64
Description: weird to_timestamp behaviour with out of range values
Details:

I would expect the to_timestamp function to return an error when I feed it
out of range values, e.g. months > 13 and days > 31. Instead it seems to add
the surplus to the timestamp and then return it.

E.g. 21-21 becomes sept. 22th the next year.

psql output:

postgres=# select to_timestamp('2011-21-21', 'YYYY-MM-DD');
to_timestamp
------------------------
2012-09-22 00:00:00+02
(1 row)

postgres=# select to_timestamp('2011-21-42', 'YYYY-MM-DD');
to_timestamp
------------------------
2012-10-13 00:00:00+02
(1 row)