showing records from the last 20 min

I know this is such a simple question, but I can't find the answer in
the manual and I've tried very hard to find it.

I want to show all the records in a table that occurred in the last 20
min.

So, maybe something like:
select * from sys_logins WHERE tstamp >= now() - '00:20';

on pg 7.3.2 this produces the error:
ERROR: Bad timestamp external representation '00:20'

Thanks for any help,
--
Matthew Nuzum
cobalt(at)bearfruit(dot)org
www.bearfruit.org