In this paper, we show the existence of function which is not S-asymptotically ω-periodic, but which is S-asymptotically ω-periodic in the Stepanov sense. We give sufficient conditions for the existence and uniqueness of S-asymptotically ω-periodic solutions for a nonautonomous differential equation with piecewise constant argument in a Banach space when ω is an integer. This is done using the Banach fixed point Theorem. An example involving the heat operator is discussed as an illustration of the theory.