  We abandon the interpretation that time is a global parameter in quantum
mechanics, replace it by a quantum dynamical variable playing the role of time.
This operational re-interpretation of time provides a solution to the
cosmological constant problem. The expectation value of the zero-point energy
under the new time variable vanishes. The fluctuation of the vacuum energy as
the leading contribution to the gravitational effect gives a correct order to
the observed "dark energy". The "dark energy" as a mirage is always seen
comparable with the matter energy density by an observer using the internal
clock time. Conceptual consequences of the re-interpretation of time are also
discussed.
