"The lack of a time stamp may seem like a random omission, but Google says it is a part of the company's plan to make Duo as simple as possible. The team was "almost maniacal" about that goal, Nick Fox, Google's VP of Communications Products, told CNNMoney last week".
Google Duo is like FaceTime but more fun - Aug. 16, 2016
Google Duo is like FaceTime but more fun - Aug. 16, 2016