Halliday & Matthiessen (2014: 407):
What has happened is that relative time — before, at or after a defined time reference — has come to be interpreted, in the semantics of English, as a kind of logical relation; a way of subcategorising events similar to the subcategorising of things, except that the latter is multidimensional (and hence lexicalised), whereas the former is based on a single semantic dimension and can therefore be expressed entirely by grammatical means.