OCTOBER: Curtain: Poirot's Last Case
Poirot's letter to Hastings reads 'October 1949'. However, Hastings has a grown-up daughter, Judith, and that shouldn't really be possible. He met Bella Duveen, his wife, in 1936. That would make Judith 12 or 13 years old - if she was conceived and born immediately afterwards. The reference to the year can't be ignored. After all, the episode is obviously set soon after the war; Styles is in decay and Hastings mentions the rationing.
The only possible solution here is to explain Judith's age by making her Hastings' adopted daughter. As always with these compromises, this isn't a perfect solution, but I think it works. I'lll try to explain:
Let's imagine that Hastings (or Bella) was sterile, and they wanted a child. They adopt Judith, an orphaned young girl of seven or eight, possibly when Hastings returns to Argentina in 1937 after his brief visit to England in the summer. This would also explain her Englishness, if she was schooled in England, and decided to go back there after the war. Hastings makes a brief return to England for Poirot's 'funeral'. Poirot doubtlessly sent Judith presents and cards for Christmas, birthdays and every other occassion, and her father is bound to have been talking about his adventures with Poirot almost non-stop, so this would explain her 'Uncle Hercule' comment. She feels as if she knows him like a member of the family.
Hastings is a kind-hearted and caring man. He would love Judith like his own flesh and blood. Also, Poirot biographer Anne Hart has suggested that Hastings might himself have been an orphaned child:
'We know that he went to Eton, but the only member of his family ever mentioned is his great-aunt Mary (...) It is possible that Hastings was orphaned at an early age, for one of the first things we learn about him is that he had 'no near relations or close friends'. Perhaps great-aunt Mary had been his guardian?' ('The Life and Times of Hercule Poirot', p. 156)
Hastings' comment after Poirot's death (in the adaptation), that Poirot had been like a father to him, also seems to support the claim that he was adopted. If we are to accept this theory, it would be even more natural for
Hastings to adopt a daughter. He knows what being an orphan entails. It would also add to him feeling so protective of her; he really wants her to do well in life.
This isn't ideal, but it's an acceptable solution to a fairly obvious chronology issue. After all, the producers could have no idea when the series began in 1989 that it would be running for 25 years within the same setting (late 1930s) and that there would then be a need to explain Hastings' daughter in the final adaptation.