The story is set in the summer of 1935 and much of it takes place on board a Douglas DC-3. But the Douglas DC-3 had its maiden flight shortly before Christmas that year and it was not introduced into service until 1936.
Although the events take place in 1935, there is a Pelican book, with its distinctive blue and white spine on Clancy's shelf. Pelicans, the non-fiction imprint of Penguin, was founded in 1937.
The events can be dated to 1935, when Perry beat Von Cramm in the French Tennis Championship. However, the final was played on 2nd June. When Mlle Giselle meets Japp and Poirot at the Surete, the calendar on the wall is Friday 5th July. Although 5th July was indeed a Friday in 1935, the action takes place over much less than 5 weeks.
Although this episode takes place in 1935, the 1946 novel "Sergeant Bigglesworth C.I.D." by W.E. Johns can be seen in Daniel Clancy's house.