When an initially quiescent, horizontal porous layer saturated with fluid experiences uniform heating from below, the time of the onset of natural convection in time-dependent, nonlinear temperature fields is predicted theoretically. The stability analysis is conducted on the basis of the propagation theory in which the thermal penetration depth is used as a length scaling factor under linear theory with the principle of exchange of stabilities. The solution for disturbance equations of similar transform considering time-de-pendent variations of disturbances is obtained numerically. The resulting correlations of the critical time to mark the onset of regular cells are derived as a function of the Rayleigh number, Prandtl number, and Darcy number. It is seen that natural convection sets in later as the Prandtl number and the Darcy number become smaller.