  We find a quantum mechanical formulation of proper time for spin 1/2
particles within the framework of the Dirac theory. It is shown that the rate
of proper time can be represented by an operator called the ` ` tempo
operator'', and that the proper time itself be given by the integral of the
expectation value of the operator. The tempo operator has some terms involving
the Pauli spin matrices, and the evolution of the proper time is influenced by
the spin state via these terms. The relation between the tempo operator and the
metric tensor is elucidated.
