Halliday (2008: 137):
There is serial time reference to past / present / future, beginning with deictic time [i.e. time relative to now] and moving on recursively from there; this is fully grammaticalised as the tense system in the verbal group, which operates in partial concord with lexical realisations of the same temporal categories. That is the underlying potential, in terms of which actual instances are construed by speaker and listener.