One characteristic interpretive technique in the discourse of customary international law is the identification of such norms as 'possibly emerging' or possibly in existence. Thus it is frequently asserted that a putative norm 'may' have or 'probably has' customary status. This hypothetical mode of analysis can give rise to the speculative construction of international obligations driven more by preference than by evidence. This speculative rhetorical technique is examined by reference to the account of temporal dimensions of the emergence of customary international law provided in the Chagos Archipelago Advisory Opinion of 2019. Here the International Court of Justice endeavoured to pin down the time of origin and path of evolution of a customary norm requiring territorial integrity in the context of decolonialisation as self-determination. This chapter engages with this ubiquitous characteristic of the interpretation of customary international law and argues that the accompanying opacity in relation to international legal norms – norms that are held to generate obligations – is to be deplored.