Say we have an electron in a position eigenstate [itex] \delta(P-x) [/itex] at point [itex] P [/itex] at time [itex] t_0 [/itex]. We also have a detector at another point Q. At [itex] t_0 [/itex], the probability of the detector registering the electron is zero. After a certain time [itex] T [/itex], the time evolution of the wave function generates a non-zero probability for the electron to be found at Q, but what can we say about the time of measurement? Is there a function [itex] f(t) [/itex] such that [itex] f(t)dt [/itex] gives the probability that the detector will register a measurement in the interval [itex] (t, t+dt) [/itex]?