  We develop a general theory of the time distribution of quantum events,
applicable to a large class of problems such as arrival time, dwell time and
tunneling time. A stopwatch ticks until an awaited event is detected, at which
time the stopwatch stops. The awaited event is represented by a projection
operator $\pi$, while the ideal stopwatch is modeled as a series of projective
measurements at which the quantum state gets projected with either
$\bar{\pi}=1-\pi$ (when the awaited event does not happen) or $\pi$ (when the
awaited event eventually happens). In the approximation in which the time
$\delta t$ between the subsequent measurements is sufficiently small (but not
zero!), we find a fairly simple general formula for the time distribution
${\cal P}(t)$, representing the probability density that the awaited event will
be detected at time $t$.
