After discussing the role of time in both General Relativity and Quantum Gravity, it is argued that even in ordinary Quantum Mechanics, the role of time is ambiguous. A rather surprising result is that measurements of the time-of-arrival of a free particle must always be more inaccurate than1/EwhereEis the kinetic energy of the particle. This inaccuracy is not related to the Heisenberg uncertainty relations. It is a dynamical relation which applies to individual measurements of a single quantity. ©1999 American Institute of Physics.