At Microsoft it was obvious how five minutes late was optimal - meetings usually dragged on past their end time anyhow, but never started early so it gave folks time to eg get to their next meeting (in person), coffee, bio break, etc.
Does Google have a culture of meetings ending on the dot with finality? I just don't see that working with human nature of "one last thing" and the urge to spend an extra few minutes to hammer something out.
It's just laughable to me to bother with a "ends five minutes early" option. It just doesn't work - you know you're not cutting into anyone's next meeting by consuming those last five minutes. But you can't know that if you push into the next half hour block - maybe they have a customer call up next that starts on time, so you have to wrap up.