ask about epoch query with datetime

Hi All,

my institution wants me to migrate from old postgresql to new
postgresql, but when i test one of the query it failed at query :
select date_part('epoch',datetime('2009-04-20
17:08:01')-datetime('2009-04-20 17:04:24'))::int;
that query runs well in previous postgresql (PostgreSQL 7.1.3). The
error message is ERROR: function datetime("unknown") does not exist
HINT: No function matches the given name and argument types. You may
need to add explicit type casts.

If i read the error message, it means the function has changed or
something. May I know the latest function so my query can run well in
my PostgreSQL 8.1.11 ?

Thanks