Yes and no. Yes, if you use the app. No, if you use the computer. For some reason the app is a little more off.
I know this because I was not sure which park we would be at that far ahead of time so I made 2 reservations. I only held them for a week or so until I could talk to my travel party and get a consensus.
I also did this when I had a mysterious ADR for SAI that was impervious to cancelation. It kept telling me it could not be cancelled online...so I tried cancelling on the computer via making a new ADR. The box popped up asking if I wanted to cancel the current one, YES! Got to the very end and...call to cancel. So I just made a reservation for a new time, about 1.5 hours after the original, and then called to cancel the old time.