We have measured the thermally stimulated decay of the dielectric modulus M(T, t) for poly(vinylacetate) by monitoring the electric field E(t) under the condition of a constant dielectric displacement for t > 0. This thermally stimulated polarization experiment is realized by cooling the sample at zero field to well below its glass transition temperature, applying a certain amount of charge, and then recording the voltage of the sample capacitor U(T, t) while ramping the temperature. The result of a temperature invariant distribution of relaxation times observed previously in isothermal experiments allows a direct translation of M(T, t) data into the variation of a characteristic relaxation time tau(T) or of a fictive temperature T-f(T) as a function of the actual temperature.