How do I activate the real-time clock on the TH8320U thermostat?

Locate and remove the tab labeled 'Remove' in the lower left corner on the back of the thermostat — this tab must be removed to activate the real-time clock. Once powered, the display will prompt you to enter the calendar date using the arrow keys, followed by pressing the Done key, and then set the current time. The thermostat is designed to keep the time and day in memory for up to ten years under normal use once the calendar is set. (Page 8)