  A controversy surrounding the "tunnelling time problem" stems from the
seeming inability of quantum mechanics to provide, in the usual way, a
definition of the duration a particle is supposed to spend in a given region of
space. For this reason the problem is often approached from an "operational"
angle. Typically, one tries to mimic, in a quantum case, an experiment which
yields the desired result for a classical particle. One such approach is based
on the use of a Larmor clock. We show that the difficulty with applying a
non-perturbing Larmor clock in order to "time" a classically forbidden
transition arises from the quantum Uncertainty Principle. We also demonstrate
that for this reason a Larmor time (in fact, none of the Larmor times) cannot
be interpreted as a physical time interval. We also provide a theoretical
description of the quantities measured by the clock.
