This article is from the Boats FAQ, by John F. Hughes with numerous contributions by others.
The time of day is broadcast on radio stations WWV and WWVH, which
transmit in the shortwave bands, on 2.5, 5, 10, 15, 20 MHz. The time is
announced every minute, and at other times there is a steady beeping.
Any shortwave receiver should be able to pick up these broadcasts - the
particular frequency you can receive will vary with location and time of
You can also hear the NIST's WWV broadcasts via the telephone. The
number is (303) 499-7111.