I have a Huawei Mate 9, and just set it to the atomic clock time a few minutes ago. The ClockSync app reported it was off by 0.06 second at that time.
However, a few minutes later when I picked up the phone and check again, it was already 3.2 seconds behind. How come it can be off by that much in a few minutes time? Doesn't the phone has an internal clock which should be only off by a few seconds in a day?