  The problem of time in quantum mechanics concerns the fact that in the
Schr\"odinger equation time is a parameter, not an operator. Pauli's objection
to a time-energy uncertainty relation analogue to the position-momentum one,
conjectured by Heisenberg early on, seemed to exclude the existence of such an
operator. However Dirac's formulation of electron's relativistic quantum
mechanics (RQM) does allow the introduction of a dynamical time operator that
is self-adjoint. Consequently, it can be considered as the generator of a
unitary transformation of the system,as well as an additional system observable
subject to uncertainty. In the present paper these aspects are examined within
the standard framework of RQM.
