 Metric temporal logics go back to Prior, too (see Prior 1967, Chapter VI). He used the notation \(Pn\varphi\) for “It was the case the interval \(n\) ago that \(\varphi\)” (i.e. \(\varphi\) was the case \(n\) time units ago) and \(Fn\varphi\) for “It will be the case the interval \(n\) hence that \(\varphi\)” (i.e \(\varphi\) will be the case \(n\) time units hence). These operators presuppose that time has a certain metric structure and can be carved up into temporal units, which may be associated with clock times (e.g. hours, days, years, etc.). If the relevant units are days, for example, the operator \(F 1\) reads ‘tomorrow’.