In this paper, we use probability theory to prove in suitable conditions the equivalence of equilibrium time correlation functions of microscopic density fields with the time correlation functions of local macroscopic density fields evolved by hydrodynamics in (approximate) phenomenological continuum theories of matter. We further discuss a useful and rigorous numerical algorithm, derived from this framework, to compute macroscopic space- and time-dependent behaviors (such as the hydrodynamical one) via molecular dynamics simulations.