When does a Measurement or Event Occur
  Within quantum mechanics it is possible to assign a probability to the chance that a measurement has been made at a specific time t. However, the interpretation of such a probability is far from clear. We argue that a recent measuring scheme of Rovelli's (quant-ph/9802020) yields probabilities which do not correspond to the conventional probabilities usually assigned in quantum mechanics. The same arguments also apply to attempts to use the probability current to measure the time at which a particle arrives at a given location. 
J. Oppenheim, B. Reznik, W. G. Unruh