In the interpretation of treaties, according to Article 31 of the 1969 Vienna Convention, interpreters shall pay primary regard to conventional language and to "relevant rules of international law applicable in the relations between the parties". Applying this provision, it is obvious that interpreters will sometimes face questions of an inter-temporal nature. What law or what language should be brought to bear on the interpretation process, exactly? The one that existed when the interpreted treaty was concluded? Or the one that exists at the time of interpretation? Over the last 30 years, international law doctrine has increasingly come to favour a rather lax approach to this issue. As the prevailing theory contends, different choices should be made depending on the particular treaty interpreted. The present article inquires into the reasons justifying this theory. As argued extensively, although there are certainly good reasons to agree with what the current legal doctrine contends, those good reasons are not the ones commonly invoked: the doctrine of inter-temporal law, as expressed in the Island of Palmas Arbitration, and the two decisions of the International Court of Justice in Namibia and the Aegean Sea Continental Shelf Delimitation, respectively. The supportive reasons lie elsewhere. As argued in the present article, it is imperative that international lawyers come to understand this; particularly if we consider it important that all inter-temporal problems – including those obviously foreseen in the international law literature – are resolved consistently.