[en] Time plays an unusual role in quantum theory, and the measurement of time is very different from the measurement of other physical qualities associated with a particle. As an example, the measurability of when something occurred is conceptually fraught with difficulties within the theory. Time must be measured by clocks, and one must somehow cause the occurrence of the event of interest to interact with a clock to record when that event occurred. But that interaction carries with it an inevitable perturbation of the event itself. I will argue that in addition to the usual ΔtΔE > ℎ associated with the accuracy of any clock, there is an additional ΔtE > ℎ uncertainty in the measurement of the time of arrival of a particle. Furthermore this constraint arises because the timing device can itself prevent the event from ever occurring at all. I will compare time measurements involving physical clocks, with attempts to construct a time operator and describe new difficulties associated with the latter approach