It wasn't based on real time. If we end a session with the party going back to keep to resupply (which happens outside of the session), you figure how much time that would take in game. 1 day to get to the keep, 1 day to resupply, and 1 day to get back. So when you start the next session, whether that happens 1 day or two weeks later, 3 days in the game passed, and creatures would react accordingly (doing their own potential reinforcements).
Never did I ever see us match in-game time with real life time between sessions. We knew the rule (contrary to the video's claim) but thought it was un-fun. So we ignored it.