  Inspired by the discrete evolution implied by the recent work on loop quantum
cosmology, we obtain a discrete time description of usual quantum mechanics
viewing it as a constrained system. This description, obtained without any
approximation or explicit discretization, mimics features of the discrete time
evolution of loop quantum cosmology. We discuss the continuum limit, physical
inner product and matrix elements of physical observables to bring out various
issues regarding viability of a discrete evolution. We also point out how a
continuous time could emerge without appealing to any continuum limit.
