timestamps, epoch and time zones

Hello,

Maybe it's a question for pgsql-novice, but I have a problem converting
to and from unix times. In particular, it seems that a round trip
unix->PG->unix does not return the original timestamp:

select extract(epoch from timestamp 'epoch');

date_part

-----------

-3600

Shouldn't that be zero? My timezone is Europe/London (but does it
matter?).

Thanks

Alberto