  We introduce a formalism for the calculation of the time of arrival t at a
detector of particles traveling through interacting environments. We develop a
general formulation that employs quantum canonical transformations from the
free to the interacting cases to compute t. We interpret our results in terms
of a Positive Operator Valued Measure. We then compute the probability
distribution in times of arrival at a detector of those particles that, after
their initial preparation, have undergone quantum tunneling or reflection due
to the presence of potential barriers. We obtain the expected retardation or
advancement for transmitted wave packets, and non-foreseen double bump
structures for some cases of reflection.
