We extend the usual gravitational action principle by promoting the bare cosmological constant (CC) to a field which can take many possible values. Variation gives a new integral constraint equation for the classical value of the effective CC that dominates the wave function of the Universe. The expected value of the effective CC, is calculated from measurable quantities to be O(t(U)(-2)) as observed, where t(U) is the present age of the Universe in Planck units. This also leads to a falsifiable prediction for the observed spatial curvature parameter of Ω(k0) = -0.0055. Our proposal requires no fine-tunings or extra dark-energy fields but suggests a new view of time evolution.