  We propose a time-of-arrival operator in quantum mechanics by conditioning on
a quantum clock. This allows us to bypass some of the problems of previous
proposals, and to obtain a Hermitian time of arrival operator whose probability
distribution arises from the Born rule and which has a clear physical
interpretation. The same procedure can be employed to measure the "time at
which some event happens" for arbitrary events (and not just specifically for
the arrival time of a particle).
