Featuring 5 Working Modes, the YL-T120V Astronomical Timer is the perfect solution for your energy saving needs. The "ON" mode is the always on setting. The "OFF" mode is the always off setting. The "AUTO" mode will utilize the built in photocell to enable the Dusk-to-Dawn feature which will automatically turn your device on at dusk and off at dawn. The "TIMER" mode will set your device to turn on during a specific time frame. The "ASTRO" mode will sync with your local area's sunrise and sunset time. The ASTRO mode is best used where lighting for your photocell is no good. The Timer comes equipped with a rechargeable 3.6V LIR2032 Lithium Ion Button Battery. Simply plug in the timer using the input plug located in the back of the device and you will be able to time two devices at a time using its two output sockets.