If you look at the bottom right of the unit, you can see the black string that is used to pull across a trail. When the string is broken the time and date is frozen on the screen. Every time that the string is broken, it has to be reset. Pretty archaic, but the alternative at the time was to sit and watch the trail yourself. First came out in 1985
Keep life simple...if you hear hoofbeats, think horses, not zebras.