Allowing SYSDATE to Work

I'd like SYSDATE to work syntactically and semantically the same as
CURRENT_TIMESTAMP (or CURRENT_TIME, or whatever). I can create a
function called "sysdate" that does the trick, but then it seems I have
to reference the function as "sysdate ()," but I want to be able to get
away with just "sysdate." It seems that CURRENT_TIMESTAMP and their
friends are magic functions that can be referenced without an explicit
empty argument list.

I have much Oracle-specific code that references sysdate, and porting
would be easier if that syntax could work unchanged in Postgres.