Adding modified and creation datetime to system catalogs

I was looking around for ways to find out the creation date of a database or
a table, but there doesn't appear to be any functions or available metadata
to provide this information. Unless there's a way I haven't seen yet, does
anyone see any problem with adding a creationdatetime and modifieddatetime
column to catalogs such as pg_database and pg_tables (but also maybe
pg_views, pg_language, pg_type etc)? Or should this be stored separately
with oids indices with associated datetiimes? I imagine it would be useful,
and I've seen this on other RDBMS' (except for modified date and time).

An example of where I was intending to use this a short while ago was to get
a list of tables and sort them by date created to review the oldest tables
for relevance.

Hopefully my suggestion is completely unnecessary and there will already be
a way to identify these pieces of information.

Thanks

Thom