converting an epoch to a timestamp

How does one go about converting a UNIX epoch (ie. int4) to a timestamp?

These don't work:

("datetime" is the int4 field)

extract(timestamp from epoch datetime)
extract(timestamp from datetime)

I'm using 7.1.3, but am also interested in 7.2 solutions.  And if it isn't
part of extract, shouldn't it be?

Chris