We are comparing against the U.S. National Institute of Standards and Time (NIST) atomic clock that broadcasts a time signal from Boulder Colorado. The signal can be received via digital clocks with a built-in radio that decodes the signal and presents perfectly accurate time, automatically synchronized once or more per day. The signal can also be received aurally via a plain old shortwave radio tuned to 2.5, 10, 15, or 20 Mhz (archaic, but perhaps more fun.)
The clocks can be purchased practically anywhere, check out Wal-Mart, RadioShack, or ebay (search under "atomic clock").
Good luck.