An appointment can be future or past - it is a fixed date.
I had an appointment for a consultation with one of my doctors two weeks ago.
However, he had to postpone that appointment to a later date. After the consultation, I made a future appointment for a follow up visit.
So far as an advanced warning, I will tell you now that one thing that gets me riled up is peoople refusing to give way to persons to the right of them. If you block me in the hallway, I will only warn you three times to yield the right of way before I move you permanently out of my way.
In other words, forewarn or advanced warning is absent the action that would get the warning.