  What are the justifications for his axioms and rules? In ‘Diodoran Modalities’, Prior was content to describe \(FFp \rightarrow Fp\) as ‘obvious enough’, but by the time of the Wellington Congress his thinking had moved forward considerably. He there set out what he called the ‘\(l\)-calculus’ (he was later to prefer the term ‘\(U\)-calculus’). ‘\(l\)’ is the relation ‘is later than’ (relating dates). In the \(l\)-calculus, the propositions of the tense calculus are treated as expressing properties of dates. ‘lxz &amp; \(px\)’ (where \(p\) might be ‘Socrates is sitting down’, for example) is read ‘\(x\) is later than \(z\) and \(p\) at \(x\)’ \((x\) and \(z\) are dates). Using the (arbitrary) date \(z\) to represent the time of utterance, \(Fp\) is equated with \(\exists x(lxz \amp px)\) (‘\(p\) at some time later than \(z\)’), and \(Pp\) with \(\exists x(lzx \amp px)\) (‘\(p\) at some time earlier than \(z\)’). \(Gp\) and \(Hp\) are equated with the universal quantifications \(\forall x(lxz \rightarrow px)\) and \(\forall x(lzx \rightarrow px)\) respectively. (Much later, Prior recast the date-terms of his \(U\)-calculi as ‘instant propositions’—propositions true at one and only one instant—so introducing what is now called   hybrid logic.)