The Activity timer screen shows you the total time in hours, minutes and seconds you were recording activity on the pedometer for a given day.
If you don't see a person on the left of your screen, the step sensor is paused and no activity time will be captured. See how to activate step sensor.
The pedometer is designed to have the screen on all the time, it won't use any more power than a digital watch when idle.
To advance from one screen to the next, press the MODE button.