Date/Time problem -(((

Hello

I am moving from MySQL to Postgres and while converting my sql code I
have a lot of problems with this construction:

($ze is current time - 300 secounds):

delete from onlineuser where datum < FROM_UNIXTIME($ze)

Is there any Date/Time function in PGSQL? I want as shown in this
example delete entries older than 300 secounds.

I postet to the list because I have found no solution in the
tutorials and search-engine (maybe searched with wrong words?)

It would be great if someone could help me / point me to the right
way...

--
Boris