From timestamp to seconds since epoch?

I've read through all the documentation, and I can't find a function
that takes a timestamp and converts iit to seconds since epoch.
Here's what I need to do: I have pairs of timestamps ('2000-07-12
03:33:53+00'), and I want to find out how many seconds are in the
interval between them. I know I can do

select timestamp1 - timestamp2

and I get a result in days, hours, mins, and seconds, but is there a
way to get that whole thing in just seconds? This has to be pretty
efficient, because it's going to get called a lot.

Thanks,

e