Clock readings are provided by two US time agencies, the National Institute of Standards and Technology,
NIST, and its military counterpart, the U. S. Naval Observatory,
USNO. These readings contribute to
world time or
UTC. Very accurate times are maintained with
atomic clocks.
UTC can be heard on Colorado shortwave station WWV at 2.5, 5, 10, 15 and 25 MHz. Or you could just bip over to www.time.gov.