https://phys.org/news/2018-06-quantum-stopwatch-memory.html

As the physicists explain in their paper, when it comes to making highly accurate time measurements, some clocks are better than others for technological reasons. But all clocks—no matter how well-constructed—are subject to a fundamental quantum limit that has its roots in Heisenberg's uncertainty principle. Due to this quantum limit, larger clocks have smaller measurement errors, but no clock can be so large that it is completely error-free.

As a result of this limit, when one or more clocks make multiple time measurements—for example, when measuring the total duration of a sequence of events—then the errors accumulate. This leads to an inaccuracy that grows linearly with the number of measurements.