When Captain Black returned from Kenya to visit Jack and Susan Maltravers at their home Marsdon Manor, he brought them an African tribal mask wrapped in a Nairobi newspaper.
The date on that paper was July 27, 1935. But that doesn't necessarily mean that the events of "The Tragedy At Marsdon Manor" (a 'Poirot' mystery) took place soon after that. First off, some time had to be involved in Captain Black's travels to reach England from Kenya: a land trip, an ocean voyage, perhaps several connecting plane flights.
And then there's the fact that the newspaper may have been laying about Captain Black's Kenyan residence for some time before he decided to use it as wrapping paper.
There's no way the chronology for the 'Poirot' mysteries can be based on broadcast order - not with "The Million Dollar Bond Robbery", connected as it is with an historical event from May of 1936, being televised before "The Plymouth Express", which occurred just after Sir Malcolm Campbell setting the land speed record in September of 1935.
But I think that even so, "The Tragedy Of Marsdon Manor" does happen soon after the episode of "Wasp's Nest", which took place in early August, 1935 and which precedes this episode in the broadcast order.
Therefore, I'm going to say "The Tragedy Of Marsdon Manor" takes place the second week of August, 1935.
And as Dennis Miller says, "Of course, I could be wrong......"