The WWV time clock is an atomic clock which keeps very accurate time based on atomic frequencies. It is named after the shortwave radio station near Fort Collins, Colorado, which broadcasts the times on the WWV frequency.
WWV is the call sign of the U.S. Institute of Standards and Technologies shortwave radio station. There are actually two such shortwave stations broadcasting the time. The second one is WWVH in Kauai, Hawaii. These stations transmit the time in two ways: an English-language recording and also a binary time code, which also gives the date. The atomic clock is more accurate than Greenwich Mean Time.