 The notions speech time, event time, and  reference time were introduced by Reichenbach (1947) in order to distinguish simple past and present perfect or, more generally, absolute and relative tense. According to Comrie (1985), who refined Reichenbach’s system (in Chapter 6 of his book on tense) speech time and event time are sufficient for the analysis of absolute time; i.e., present, (simple) past and future. But for relative tense—of which the present perfect is but one example—reference time is required. Let us explain these notions by applying them to distinguish between past and present perfect. 