Abstract
We analyze the nature of the Schrödinger equation and of the Dirac equation. Due to the theoretical reversibility of time (see Olivi-Tran [1, 2, 3, 4]), a simple transformation of Scḧrödinger equation leads to that conclusion. In the case of Dirac equation the nature itself of the Dirac matrices leads to a quantization of time when solving this equation with the associated matrices.