how to use function from different database

Hi,

I created a generic function (DATEFIFF) login to postgres database using
psql. ANd I wanted to use across the entire instance , from all other
databases.

postgres=> CREATE FUNCTION DateDiff (units VARCHAR(30), start_t TIMESTAMP,
end_t TIMESTAMP)
postgres-> RETURNS INT AS $$
postgres$> DECLARE
postgres$> diff_interval INTERVAL;
...
...

grant execute on function DATEDIFF to public;
grant execute on function DATEDIFF(varchar,timestamp,timestamp) to public;

I tried both the grant one at a time, none of them works.
------------------------------------------
Connected to another database
\c application1

application1=> SELECT * from DATEDIFF('second', '2011-12-30
08:54:55'::timestamp, '2011-12-30 08:56:10'::timestamp);
ERROR: function datediff(unknown, timestamp without time zone, timestamp
without time zone) does not exist
LINE 1: SELECT * from DATEDIFF('second', '2011-12-30 08:54:55'::time...
^
HINT: No function matches the given name and argument types. You might
need to add explicit type casts.
application1=>
------------------------------------------

How can I use this function across my entire postgresql instance from any
databases ?

--
Thanks.
Bhupendra B Babu