We present a new approach to static program analysis that permits each expression in a program to be assigned an execution time estimate. Our approach uses a {\it time system} in conjunction with a conventional type system to compute both the type and the time of an expression. The {\it time} of an expression is either an integer upper bound on the number of ticks the expression will execute, or the distinguished element {\tt long} that indicates that the expression contains a loop and thus may run for an arbitrary length of time. Every function type includes a {\it latent time} that is used to communicate its expected execution time from the point of its definition to the points of its use. Unlike previous approaches a time system works in the presence of first-class functions and separate compilation. In addition, {\it time polymorphism} allows the time of a function to depend on the times of any functions that it takes as arguments. Time estimates are useful when compiling programs for multiprocessors in order to balance the overhead of initiating a concurrent computation against the expected execution time of the computation. The correctness of our time system is proven with respect to a dynamic semantics.