 The logic of the expressions “tomorrow” and “yesterday” used in section 1 was in fact formalized by A.N. Prior (1914–69) who suggested that the two terms should be treated as propositional operators, \(T\) and \(Y\) (1967, p. 67 ff.). This means that \(T(p)\) and \(Y(p)\) should stand for the propositions “tomorrow it will be the case that \(p\)” and “yesterday it was the case that \(p\)”, respectively. Prior even suggested some basic axioms for the two operators 