Poynting's vector theorem P=ExH is one of the universal laws of physics that applies to electromagnetic fields in AC and DC circuits. A rigorous analysis of two arbitrary cases of DC and AC circuit electromagnetic fields shows that Poynting's law P=ExH applies to both station ary and time-varying electromagnetic fields. Keeping the generality, we analyze two simple cases of time-varying and stationary fields of a regular cylindrical wire carrying direct or alternating current where in both cases the electromagnetic energy flow calculations validate the hypothesis that the theorem of Poynting is absolute. Moreover, the interpretation of the results also suggests that the photons of light beams or electromagnetic field bundles cannot live forever, they do not have an infinite but a finite lifespan because they can be created or annihilated during their interaction with free or bound charges such as electrons.