converting PostgreSQ- timestamp to UNIX-timestamp ?

Hi everybody

To convert PostgreSQL timestmp to UNIX timestamp I use

select ((today::abstime)::int4) from table ...

In the documentation there is a notice that abstime shouldn't be used.
Is there another way to convert?

Also I tried

select timetz_hash(today) from table ..

and got negative values for some timestamps. Does anybody know why?

RedHat 7.3, PostgreSQL 7.2.1


Regards
Conni