Between ‘Time Immemorial’ and ‘Instant Custom’: The Time Element in Customary International Law

Article 38.1(b) of the Statute of the International Court of Justice, which refers to customary international law as ‘a general practice accepted as law’, makes no mention of duration. Yet the ‘time element’, as the Court itself called it, has not infrequently been relevant—if not central—to determining whether a rule of customary international law has come into being. The present article seeks to describe how far the passage of time is necessary for the creation of rules of customary international law, and the possible significance of time to the customary process more generally. While noting that no particular duration is required for the formation of customary international law, it suggests that some time must always elapse, and that assertions of a rapid development in customary international law are to be treated with a degree of caution. Light is thrown on particular ways in which time may indeed be of significance for the formation and identification of a rules of customary international law, and on further ways in which time (and timing) may impact the life cycle of such rules, including their possible change and demise.