  The calculus of tenses presented in ‘Diodoran Modalities’ is the system produced by adding the following axioms, rules, and definition, to the ordinary propositional calculus. \((1)\ F(p \vee q) \equiv(Fp) \vee  (Fq)\). \((2)\ FFp \rightarrow Fp\). (3) If \(p \equiv q\) is provable in the calculus (or is an axiom) then \(Fp \equiv Fq\) follows. (4) If \(p\) is provable in the calculus (or is an axiom) then \(Gp\) follows; where \(Gp\), the future perpetual form ‘It will always be the case that \(p\)’, is defined as \(\neg F\neg p\).