Make year 01/01/0001 but leave timestamp alone - Mailing list pgsql-novice

I have a situation where we have a timestamp column but in special cases we want to set the date to '01/01/0001' and leave the timestamp alone. For example, '11/04/2005 10:30:05' would become '01/01/0001 10:30:05'. I've been going through the various date time functions but don't see a simple way to do this. Can someone help?

Thanks,

Jed