There is a tension in the doctrine of intertemporal law outlined by Max Huber in the Island of Palmas case. The first branch demands that the legality of an act be judged by the law in force at the time the act occurs. The second that we take into account any change in the law over time. We see the problem in the 2019 Chagos Archipelago proceedings. The UK argued that the detachment of the Archipelago from Mauritius in 1965 was not unlawful, because it was not regarded as unlawful at this that time. The International Court of Justice (ICJ) disagreed, deciding that the detachment was unlawful at that time, but it relied on the 1970 Declaration on Friendly Relations to confirm this conclusion. This article explains why the ICJ’s use of dynamic logic to reach its decision was correct – —and what this tells us about the intertemporal doctrine.