Halliday & Matthiessen (1999: 215):
English foregrounds location in the flow of time (tense), and construes this not only as past/present/future relative to ‘now’ […] but also as past/present/future relative to some moment that is relative to now […], with the possibility of up to five shifts of reference point, as in
They said they’d been going to’ve been paying me all this time …(present in past in future in past in past). This system is fully grammaticised, and is unusual in that it construes location in time as a logical relation rather than as an experiential taxonomy; it thus becomes a form of serial time reference. The tense categories also combine with time adverbs such as already, just, soon […]. Interestingly, the deictic time reference (that appealing to ‘now’) can be switched off; either there is no deixis (the clause is non-finite […]) or the deixis takes the form of modality (speaker’s angle on the process, eg they should have paid me).