We present a redshift--time framework to study the comoving density of planetary systems that could, potentially, harbor life at an observation epoch. We adopt a flat LambdaCDM cosmology and combine a cosmic star-formation rate density with an initial mass function to compute the density of surviving host stars via a lifetime-based survival indicator. We then model physical habitability as the product of planet-centric factors (e.g., habitable-zone residence and rocky composition) and an environmental survival term expressed using hazard rates (e.g., gamma-ray bursts and AGN activity). Finally, we add a module that couples generalized stoichiometric physiology to a chemostat-style biogeochemical ODE system for nutrient--biomass dynamics, yielding an additional filter for the fraction of physically habitable planets that could sustain an active aqueous biosphere capable of producing chemical signatures.