MAY: How Does Your Garden Grow?
A ticket says “Chelsea Show 1935, Vincent Square London SW1”, with the dates Wed 22nd, Th 23rd, Fr 24th of May. Also, a banner outside the Chelsea Show states "The Chelsea Flower Show 1935". In other words: May 1935.
JUNE/JULY: Death in the Clouds
Poirot hands Japp a paper dated ‘Vendredi 5 Juillet 1935’. The same date is shown in the calendar of Inspector Fournier's office.
JULY: The Adventure of the Clapham Cook
The date July 11th 1935 is given on the cheque from Mr. Ernest Todd.
JULY: The Veiled Lady
The housekeeper of Mr. Lavington mentions that Fred Perry won again last year. That means it could be placed either in 1935 or 1936 (according to Wikipedia). I choose 1935 for chronology reasons. Month: July. Hastings states that “anyone might have used the fire”, to which Poirot answers: “In July? I don’t think so”.
AUGUST: The Lost Mine
The man pretending to be Mr. Wu Ling writes the date in the reception book (8/2/35). There is also a cheque dated "27th July 1935" (see below).
I had previously placed this in February, but as Michael very rightly points out in a comment to this post, the date is in August. I take the liberty of quoting him here:
The man checking into the hotel pretending to be Wu Ling uses the American format of placing the month first. This is clear from the date above which is written as '2nd Aug'. Poirot later comments on this, as the date format lets him know that the man checking-in had an American accent (he goes on to say Americans are a "very backward people"). So the episode takes place in August, which makes the cheque at the end of the episode dated 27 July 1935 consistent (Miss Lemon confirms it should have been paid in to Poirot's account 'ten days ago').How I managed to overlook this I really don't know. But I am very grateful for being made aware of the mistake!
