TIME IN CONSUMER THEORY Eric English Boulder, CO | June 20, 2020 Abstract Consumer theory is extended to allow for the rigorous treatment of time in utility maximization. Consumer theory traditionally evaluates a marginal change in the amount of a given good, but a marginal change in the amount of a given use of time is undefined. Instead, marginal changes for time necessarily involve an exchange of two uses of time, represented by the difference of two utility derivatives in the mathematics of utility maximization. A mistaken belief that single utility derivatives for time are economically meaningful has led to widespread errors in interpreting and estimating time values.