  Clocks in different heights or with different velocities run with different
speeds. For global positioning systems these effects are much too large to be
ignored. Nevertheless, in classical and quantum mechanics we get high accuracy
using a "universal" time scale, not depending on altitudes and velocities. One
may ask how this is possible. The answer to this question we may get from the
observation that in classical and quantum mechanics time and energy are
canonically conjugate variables. We argue that the mentioned modifications of
the time scale by relativistic effects are taken into account in the notion of
energy. On the basis of the experimental results and the laws of special
relativity we argue that we should consider energy as measure for the elapse of
time.
