When fashion model Molly Deane drove out to see her boyfriend, the author John Harrison, she brought along a copy of Vogue which featured her on the cover.
The date on the magazine was September 10th, 1935.
As magazines are published well in advance of their cover date so that they have a longer life at the news stands, this issue of Vogue probably came out in mid-August. Molly must have had an advance copy before its release, as it was treated as if it was a big surprise.
Therefore, I think the 'Poirot' episode "Wasp's Nest", in which this all occurs, takes place in the first week of August, 1935.