Halliday (2008: 135):
But the tense system of English is organised on this serial principle: each choice of past, present or future may serve as a point of reference for a further move into past, present or future, and this can happen four or five times over. There is no requirement that each step should be made explicit; the speaker can move in at any point, and verbs with up to three tense choices are commonly used as dialogic openings, for example
- I’ve been going to ask you: … future in past in present
“the present situation is that in the past I was contemplating the future”.