  The kinematic time operator can be naturally defined in relativistic and
nonrelativistic quantum mechanics (QM) by treating time on an equal footing
with space. The spacetime-position operator acts in the Hilbert space of
functions of space and time. Dynamics, however, makes eigenstates of the time
operator unphysical. This poses a problem for the standard interpretation of QM
and reinforces the role of alternative interpretations such as the Bohmian one.
The Bohmian interpretation, despite of being nonlocal in accordance with the
Bell theorem, is shown to be relativistic covariant.
