  The observed cosmic acceleration was attributed to an exotic dark energy in
the framework of classical general relativity. The dark energy behaves very
similar with vacuum energy in quantum mechanics. However, once the quantum
effects are seriously taken into account, it predicts a completely wrong result
and leads to a severe fine-tuning. To solve the problem, the exact meaning of
time in quantum mechanics is reexamined. We abandon the standard interpretation
of time in quantum mechanics that time is just a global parameter, replace it
by a quantum dynamical variable playing the role of physical clock. We find
that synchronization of two spatially separated clocks can not be precisely
realized at quantum level. There is an intrinsic quantum uncertainty of distant
clock time, which implies an apparent vacuum energy fluctuation and gives an
observed dark energy density $\rho_{de}=\frac{6}{\pi}L_{P}^{-2}L_{H}^{-2}$ at
tree level approximation, where $L_{P}$ and $L_{H}$ are the Planck and Hubble
scale cutoffs. The fraction of the dark energy is given by
$\Omega_{de}=\frac{2}{\pi}$, which does not evolve with the internal clock
time. The "dark energy" as a quantum cosmic variance is always seen comparable
with the matter energy density by an observer using the internal clock time.
The corrected distance-redshift relation of cosmic observations due to the
distant clock effect are also discussed, which again gives a redshift
independent fraction $\Omega_{de}=\frac{2}{\pi}$. The theory is consistent with
current cosmic observations.
