Old quantum postulate was E= h w, where w= 2 pi/ t. Nowadays, we start from the (afaik) equivalent postulate [x,p]=ih. I suposse this was originated by Dirac-Poisson methodology. One wonders, Which are the merits and disadvantages of each postulate? For instance, the old one seems more physical, but I have never seen a quantum field theory formulated from the old principle. Also, it is very intriguing to see how to fit both postulates into the path integral formulation.