Reference time

Definition
Reference time is the third time point R in Reichenbach's theory of tense. This point is used next to the points S (speech time) and E (event time) to represent the meaning of tenses. The relevance of R can be seen most clearly in the different representation of simple past and present perfect:

simple past		I saw John ...---R,E -S-... present perfect		I have seen John ...---E-S,R-...

Given that the reference point can be interpreted as the time from which an event is 'seen', the difference is that in the simple past the event is seen from some moment in the past, while in the present perfect the event is seen from the present.

Links
Utrecht Lexicon of Linguistics