The problem of time in quantum mechanics (QM) concerns the fact that in the Schrödinger 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 an electron’s relativistic QM 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 relativistic QM.