I propose a general geometric framework in which to discuss the existence of time observables. This frameworks allows one to describe a local sense in which time observables always exist, and a global sense in which they can sometimes exist subject to a restriction on the vector fields that they generate. Pauli’s prohibition on quantum time observables will be derived as a corollary to this result. We will then see how time observables can be regained in modest extensions of quantum theory beyond its standard formulation. http://arxiv.org/abs/1404.4961