The US Atomic Clock is the most accurate clock in the world and the official national timepiece. This clock is operated by the National Institute of Standards and Technology (NIST) in Fort Collins, Colorado. Using a low frequency radio transmission from NIST, the US Atomic Clock sends an accurate time signal across the continental United States. The SkyScan® Atomic Clock automatically sets itself to the correct time and date by receiving and decoding the NIST radio signal with an internal receiver. It ensures daily accuracy to the fraction of a second by refreshing the signal each day. Awesome clock in beautiful preowned condition. No battery back cover. Batteries not included. The item will be carefully boxed and wrapped so that it arrives in pristine condition. Thank you so much for considering my listings!