Special Date/Time Inputs

Hi
How do you use Special Date/Time Inputs described in Table 5.14 
(Postgresql documentation)?
It looks like only now() works. When I try to select epoch() I get 
error, that such function doesn't exist.

Anyway it came from another problem:
I have table with millions records indexed on timestamp. I tried to 
speed up using this table, so I changed timestamp into 
seconds-since-epoch (integer). What do you think about it?
Is there a better way to convert this field back into timestamp than 
'1970-01-01'::timestamp+cast(field || ' seconds' as interval) ?

Regards,
Tomasz Myrta