Software-Defined Networks (SDNs) introduce interesting new opportunities in how network routes can be defined, verified, and changed over time. Despite the logically-centralized perspective offered, however, an SDN still needs to be considered a distributed system: rule updates communicated from the controller to the individual switches traverse an asynchronous network and may arrive ... [Show full abstract] out-of-order. This can lead to (temporary or permanent) inconsistencies and triggered much research over the last years. We in this paper initiate the study of algorithms for consistent network updates in “timed SDNs”—SDNs in which individual node updates can be scheduled at specific times. While technology enabling tightly synchronized SDNs is emerging, the resulting algorithmic problems have not been studied yet. This paper presents, implements and evaluates Chronus, a system which provides provably congestion- and loop-free network updates while avoiding the flow table space headroom required by existing two-phase update approaches. We formulate the Minimum Update Time Problem (MUTP) as an optimization program and propose two polynomial-time algorithms which lie at the heart of Chronus: a decision algorithm to check feasibility and a greedy algorithm to find a good update sequence. Extensive experiments on Mininet and numerical simulations show that Chronus can substantially reduce transient congestion and save over 60% of the rules compared to state of the art.