Exact time keeping is a small obsession around here. A decade or more ago, I got an HP Z3801A GPS
Receiver from E-Bay and it has worked very well. The point of this thing is not to tell
you where you are, but to tell you what the time is. More specifically, it produces a
10MHz frequency reference signal which is typically accurate to within 1 part in a billion, along
with a 1 pulse per second signal that is synchronized to GPS time to better than a microsecond.
These signals can be used to drive other things - such as frequency synthesisers, counters,
clocks, time servers ...