In this link you will see that this ELL book does not change the words of time
I joined the company three years ago
The reported speech version is:
He said that he had joined the company three years ago
I checked Swan's PEU section 274.4 and didn't find any comments about whether it is optional or necessary to change these words. I understand that it is optional to backshift if the situation is still true. But once you opt to backshift I believe you should change the time words too and not just the verbs. Am I correct?
And on another page of the same book mentioned above, they didn't change the future words either.
We can deliver next week
Reported version:
He said (that) they could deliver next week
Is there a rule one could use?
Best Answer
The 'rule' is that you change temporal references if the facts require it.
For instance, ago designates a timespan whose end lies at ‘Speech time’ (ST), the time a speech is uttered. If the STs of the speech and the report lie within the same timeframe, there is no need to adjust the reference:
But if the STs of the speech and the report lie within different timeframes, the reference may have to be adjusted.
Other terms which refer to ST Terms—tomorrow, yesterday, last year, next week—similarly require adjustment when STs get out of synch.