There are a number of ways in which time affects treaty interpretation. This chapter examines how a text can be understood, in Roland Barthes’s famous expression, as a ‘weapon against time’ (i.e. a device of stability). In international law, such understanding of a text as a safeguard against change is rooted in the principle of contemporaneity, according to which the terms of a treaty shall be interpreted according to the meaning prevailing at the time of its conclusion. Nevertheless, an exception to this principle has emerged in the form of the doctrine of evolutionary interpretation. This chapter thus looks at the ways in which lawyers navigate the tensions between stability and the perceived need for law to respond and adapt to changing social circumstances and present-day conditions. The chapter then turns to the question of subsequent practice and, more precisely, what should be meant by ‘practice’. It also seeks to illuminate the stakes of the debate about the ultimately rejected draft Article 38 of the Vienna Convention, which provided for the possibility of the modification of a treaty by subsequent practice. Finally, the chapter also broaches the question of the boundary between interpretation, law-making, and the emergence of a new rule of customary international law.

This chapter will focus on two issues discussed broadly by investment tribunals: rules on conflicts of treaty norms (Article 30 of the VCLT and corresponding CIL) and rules relating to subsequent agreements in relation to the interpretation of treaties (Article 31(3)(a) of the VCLT and corresponding CIL). With respect to the conflicts of treaty norms - a coherent line of tribunals decisions concerning relations between international investment agreements and EU treaties is introduced, which narrowly interpret the issue of ‘the same subject matter’. Regarding the rules on subsequent agreements several observations are made. First, the ease with which arbitral tribunals reject interpretations formulated by States through subsequent agreements is striking. Second, tribunals have set out detailed requirements for the formulation of a declaration, without explaining the source for these requirements. Third, there is a considerable difference between tribunals referring to joint interpretations of State Parties, as envisaged by the treaty as binding (such as with NAFTA), and the tribunal assessing such interpretations through the lens of the VCLT or corresponding customary rules.