A Mathematical Definition of the Present and its Duration Let τ be a real variable that runs from a selected system's future into its present and then into its past a la McTaggart's A-series. We may define a unit of becoming, e, that coordinatizes τ the way seconds coordinatize McTaggart's B-series earlier-times to later-times (e is not the electric charge in this context). By convention we will suppose that τ > 0 means the (A-series) time is in the selected system's future, τ = 0 is its present, and τ < 0 its past. One doesn't need to make the sizable assumption the present is a single infinitesimally small point centered at, for example, τ = 0. (It may be the smallest duration is the Planck time anyway.) Define for each τ a 'degree presentness' p = p(τ), so the present may be spread out in A-series time somewhat. (Smith, 2010). By convention we will suppose p(τ) =1 means that τ is fully present, p(τ) = 0 means that τ is fully not present (thus either in the future or the past of the selected system), and 0 < p(τ) < 1 means that τ is partially part of the present. One may consider symmetric functions p, asymmetric functions p, step functions p, infinite-tailed functions p, normalized functions, etc. It would be philosophically dubious to have a disconnected function p. The block-world theorist would have p(τ) = 1 for all τ. The growing-block theorist would have p(τ) = 1 for τ ≤ 0. The presentist (like me) would suppose τ is at least partially present where p(τ) > 0 (i.e. on the support of p). Suppose for the sake of argument that an A-series is associated with each physical system the way qualia are associated with each physical system in Panpsychism. Then it may be that one system has a presentism function p(τ) whereas a different system has a different presentism function p'(τ'). If for two systems p(τ) and p'(τ') are non-point-like then there would be some uncertainty as where in the present τ'' an event or process is if these two systems come to have the same A-series. So there would seem to be some kind of uncertainty relation here. For each selected system there are five not four variables, τ the A-series, t the B-series, and the three space dimensions xa for a = 1, 2, 3. References forthcoming