"The referee may correct an obvious mistake by the timer to start or stop the clock properly only when he/she has definite information relative to the time involved." Rule 5-10-1
OK, let's set the issue of "definite knowledge" aside. The only way I could approximate how much time lapsed would be to have the timer start the clock, then immediately stop it. Note that the case Bob cited reads "1/10s," i.e., plural.
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