Heterochrony is the evolutionary change of rates and timing of developmental processes. Many artificial evolutionary development systems have the ability to produce heterochrony implicitly. However, few systems have the ability to employ heterochronic change as direct evolutionary means, i.e. to directly change heterochronic features of the developmental program. In this paper, we present and investigate a system that can directly modify heterochronic features of its basic dynamics. We show how heterochrony can be included in a novel developmental framework called vector field embryogeny, and investigate the benefit of direct evolution of heterochronic change for four different cellular differentiation tasks.