A receiver for WWVB at 60KHz that would decode the signals would be
as accurate as anyone could want. That's the signal that your "atomic"
clock receives, so the clock *should* be accurate. You might only be
able to receive the signal in the nighttime hours, like the clock.
Typically, the clock will sync up at about 2AM local time, and a WWVB
receiver would basically do the same thing, with an accurate crystal
oscillator as an internal reference to keep time when no signal is
being received. Such a receiver would probably cost over $1000,
at a guess, but that's the difference between a scientific reference
and a $20 "atomic" clock!