When was the first Poirot story written?

"The Mystery at Styles" (a manor in rural England). It was set towards the end of WW1, in a village where British troops trained before going abroad and where a group of Belgian refugees lived thanks to the kindness of a local banefactor. She is a rich woman in her sixties, living in the respective manor with her ward, her two sons, her daughter-in-law, her second (and much younger) husband and her closest friend. One night she dies mysteriously, and Capt. Hastings (a friend of one of the sons) suggest they call in Monsieur Hercule Poirot, a pre-war acquaintance of his.