You use a jittery GPS PPS signal to discipline a highly stable primary clock through a very "slow" filter. The GPS PPS might have an RMS period jitter of say 10

You use a jittery GPS PPS signal to discipline a highly stable primary clock through a very "slow" filter. The GPS PPS might have an RMS period jitter of say 10-20 nanoseconds, but the drift will be 0. Then, you can use that disciplined primary clock to provide actual time for timestamping, etc.