Blank the screen and flash the time at a fixed interval, so you glance instead of stare.
Click the hour, minute or second digits to choose which one arrow keys, the scroll wheel or a swipe change, and to type its number: two digits fill the unit and carry on to the next (Enter keeps it, Esc cancels). The click never starts or pauses the clock. Start and pause always go through the Start button or Space. The length can only be changed while paused; pause first if the timer is running. While it is hidden, a tap peeks at the time without changing anything.