Time is set during production and reserved with a button cell battery. Battery backup feature stores time and alarm time automatically during power failure. Alarm will be activated at the scheduled time, ensuring to wake you up the next morning. Clock should be plugged in wall outlet for use. 5V/2.1A adapter is provided and the adaptor cord is 1.5 meter in length. Eight time zones optional (EST, NST, AST, HAT, AKT, PST, MST and CST). 6 levels adjustable brightness plus auto dim function. Display brightness is 100% from AM 7:00 TO PM 9:00, 5% from PM 9:01 to AM 6:59. With only batteries powered, you are not able to adjust the brightness.