System function $sys.func.IS_PAST_DATE not working for conditional route

[Sky4]

I am trying to create a route that triggers if the session time is greater than x minutes in DialogFlow CX. I have a conditional route with customized expression $sys.func.IS_PAST_DATE($session.params.endtime) and it is not working. The parameter endtime was set at the beginning of the session with $sys.func.ADD_DATE($sys.func.NOW(), X, "MINUTES") and I verified it is correct. It just seems function IS PAST DATE is not returning true or false as expected.