Note that, from the point of view of modal logic, there are, formally speaking, two different accessibility relations involved here: an ‘earlier-relation’ in the case of the past operators and a ‘later-relation’ in the case of the future operators. In temporal logic, these two relations are uniformly captured by a single precedence relation; after all, ‘earlier’ and ‘later’ are just converses of each other (i.e., \(t\) is earlier than \(t'\) iff \(t'\) is later than \(t\)).