Abstract: The interval between first and second events, which may occur in a period less than a nanosecond interval, is determined with a square wave clock source started in response to the first event. The source derives a square wave having a first transition when started and equally spaced succeeding transitions spaced from each other by T/2, where T is the square wave period. A delay network having N taps is responsive to the square wave, whereby (N+1) square waves are derived. The taps are spaced from each other so the clock source square wave is delayed at tap k by kT/n, where n is a predetermined integer greater than 1 and k is selectively every integer from 1 to N. (N+1) memory elements respectively respond to the (N+1) square waves to generate a signal indicative of which half cycle of each of the (N+1) square wave is being derived when the second event occurs.