9.2 timestamp function syntax error

I don't understand the error resulting from the following progression on
9.2 (specifically "EnterpriseDB 9.2.1.3 on x86_64-unknown-linux-gnu,
compiled by gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-52), 64-bit"):

select sysdate => timestamp without time zone

select timestamptz(sysdate) => timestamp with time zone

select timestamp(timestamptz(sysdate)) => ERROR: syntax error at or
near "timestamptz"

OR

select timestamp(sysdate::timestamptz)) => ERROR: syntax error at or
near "sysdate"

I see a function in pg_catalog with signature timestamp(timestamp with
time zone). Why isn't it being applied?

Thanks.

--
Guy Rouillier