select extract and subqueries

I don't know if its possiable or not but I've been having problems trying to extract the timestamp out of one of my columns so that I can compare the dates.

if I do the following it gives me a proper epoch
en=# SELECT EXTRACT(EPOCH FROM TIMESTAMP '2001-11-20');
date_part
------------
1006232400
(1 row)

but if I try to use a subquery to get the date it gives me a parse error.

en=# SELECT EXTRACT (EPOCH FROM TIMESTAMP (select date from master));
ERROR: parser: parse error at or near "select"

is this simply just a limitation of the extract funtion or am I missing something?

Mike