  In this article we study the nature of time in Mechanics. The fundamental
principle, according to which a mechanical system evolves governed by a second
order differential equation, implies the existence of an absolute time-duration
in the sense of Newton. There is a second notion of time for conservative
systems which makes the Hamiltonian action evolve at a constant rate. In
Quantum Mechanics the absolute time loses its sense as it does the notion of
trajectory.
  Then, we propose two different ways to reach the time dependent Schr\"odinger
equation. One way consists of considering a "time constraint" on a free system.
The other way is based on the point of view of Hertz, by considering the system
as a projection of a free system. In the later manner, the "time" appearing in
the Schr\"odinger equation is a linear combination of the time-duration with
the "time" quotient of the action by the energy on each solution of the
Hamilton-Jacobi equation. Both of them are based on a rule of quantization that
we explain in Section 4.
