"In Indirect Speech [...] the past tense in the reporting verb tends to make the verb of the subordinate clause past tense as well. This phenomenom, known as backshift, is normally optional [...]" (my emphasis added).
Leech (2004.108) writes, of avoiding backshifting:
"The implication of this avoidance of backshifting is that the time of the original utterance ('then') and the time of the of report ('now') are both included within the time-span during which the statement in the reported clause remains valid."
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