You could do this manually if you're the host and you're available at the start time, by using the setting that requires the host to manually start the game.
The issue is that you can't. Since some host were abusing Manual Start and RT games (starting the game at an arbitrary time, booting everybody and get a free win). So the option "Start game when host clink start game" is no longer an option for RT games.
Thus, there is one way, right now to make RT games start at a designated time. Make a RT tournament. Con, is that it is a tournament.
EDIT: What Dan suggested should work, however it has the potential for abuse. So players probably won't join the game.
Edited 4/29/2017 17:22:19