  The K$\acute{\text{a}}$rolyh$\acute{\text{a}}$zy uncertainty relation
indicates that there exists the minimal detectable cell $\delta t^{3}$ over the
region $t^3$ in Minkowski spacetime. Due to the energy-time uncertainty
relation, the energy of the cell $\delta t^3$ can not be less $\delta t^{-1}$.
Then we get a new energy density of metric fluctuations of Minkowski spacetime
as $\delta t^{-4}$. Motivated by the energy density, we propose two new dark
energy models. One model is characterized by the age of the universe and the
other is characterized by the conformal age of the universe. We find that in
the two models, the dark energy mimics a cosmological constant in the late
time.
