type casting reference?

hi.
i need to cast from timestamp to seconds (in the form of integer).

i have a table with a column:
expiretime timestamp

i have tried the following, it gives errors as below:
SELECT expiretime::float4 AS timeleft FROM table WHERE something='1234';
ERROR: cannot cast type timestamp without time zone to real

SELECT expiretime::text::float4 AS timeleft FROM table WHERE something='1234';
ERROR: invalid input syntax for type real: "2005-06-12 21:21:24.368082"

postgresql 7.4.1 (i know, old)

so, my question is simply, where in the docs can i find how i should perform the
cast?
it would help a lot if there was a table showing which types can be cased into
which, and possibly an example or two for situations like this, casting from
timestamp (to timestamp worked right away).

/stig