I just might make a complete ass of myself here, but here's an attempt at trying to pinpoint a date for the writing of the final two Christie novels.
There's not a whole lot of in-text evidence in the novel that could leading anyone to find its place in the chronology, but it does feature both Hastings and George and assumes them to be well-known to the reader. As Hastings had been in almost every other novel with Poirot and certainly could be assumed to be a well-known character, that doesn't help us at all.
George, on the other hand, generally didn't appear in any story where Hastings was a character. Since he was generally featured as a quick sounding-board to Hercule Poirot, that's hardly surprising. But if my reasearch is correct, George was first featured in the novel The Mystery of the Blue Train and the short stories Murder in the Mews and The Under Dog, apparently all written around the same time in the mid to late 20s.
Then he appeared again in a number of stories from The Labours of Hercules, mostly first published in 1939-40, the short story Four and Twenty Blackbirds and also in the novel One, Two, Buckle My Shoe from 1940. This indicates that as a character, George would be fairly fresh in Christie's mind during the early years of WWII, when Curtain was supposedly written.
One more thing that supports my assumptions is the fact that there is no mention at all of Ariadne Oliver in Curtain. As she became such a frequent supporting character to Poirot, and even appeared on her own in one novel, I think we can take it for given that Christie would have featured her in some capacity, had she known that Mrs. Oliver would be as prominent as she became. And as Mrs. Oliver had her second Poirot appearance in Mrs. McGinty's Dead, in the early 50s, this also supports the claim that Curtain is from the early to mid 40s.
There are also a few references to previous cases in Curtain. The last published work of that kind is Sad Cypress from 1940. The reference is to the hard-to-find murder motive in the case of Evelyn Carlisle. Since the character was renamed Elinor in the final version of the story, that seems to indicate that the Sad Cypress story had not been completely finished when Curtain was being written.
Knowing that Christie's schedule was to publish two novels a year with a helping of short stories at this point in time, it is telling that 1943 features only one novel and no short stories at all. Add to that the following statement from Christie in 1942 to her agent where she asked him to keep a manuscript in reserve:
I have been, once, in a position where I wanted to write just for the sake of money coming in and when I felt I couldn't – it is a nerve wracking feeling. If I had had one [manuscript] 'up my sleeve' it would have made a big difference. That was the time I had to produce that rotten book The Big Four and had to force myself in The Mystery of the Blue Train.
While Wikipedia says that this request probably referred to Sleeping Murder, I think it's just as plausible that it could apply to Curtain instead. Therefore, my conclusion is that it was written around 1940-42, which corresponds to the general perception.
Unlike Poirot and Curtain, where the time of writing the novel does not seem in dispute, Sleeping Murder seems harder to pinpoint. The novel itself has no air of finality about it, instead appearing to be just any one of Miss Marple's cases.
In the text, there are two important facts that help to set boundaries for when it might have been written. First, there is a reference to a previous case of Miss Marple's, the poisoned pen letters in Lymstock. This is quite obviously the novel The Moving Finger, published in 1941. That gives us the earliest possible year for the writing of Sleeping Murder.
The second in-text fact is the mention of Colonel Bantry as being still alive and still living at Gossington Hall with his wife Dolly. We were told of his death and the fact that Mrs. Bantry had moved out in The Mirror Crack'd from Side to Side in 1962. That gives us a definite final possible year for the time of writing. But in fact, I think it indicates that Sleeping Murder was finished quite some time earlier than 1962, because if Christie had known that she was going to kill off Colonel Bantry, then why use him as a living character in Sleeping Murder at all? It's not as if his appearance there is important to the story.
The Wikipedia page on this novel has a good runthrough of the different arguments regarding this novel's writing, and shows that there is clearly some merit to the assumption that Christie began writing the novel at around the same time as Curtain. Miss Marple is still fairly sprightly in this novel, working hard in her garden, which was expressly forbidden her in the aforementioned Mirror Crack'd, where she even had to have a live-in help, and gardening had gradually been shown to be something that was increasingly difficult for her in the 50s novels. This all points to a time of writing that is somewhat concurrent with Curtain. There is also the odd mention in Christie's correspondence of the fact that she is writing two novels to put aside for future use from the time of WWII.
However, as John Curran finds in his research into Christie's notebooks, we also know that there are several references to a novel in the planning stages, a novel which cannot be any other than Sleeping Murder. There are references to the novel's working title, Cover Her Face, as well as clear descriptions of certain events and passages in the novel. These references stem from 1947 and 1948.
It's also been pointed out that it seems somewhat silly to write a final novel featuring Miss Marple at a time when she had only been featured in one novel and one short story collection - exactly the same as Tommy and Tuppence, and less than Inspector Battle, who'd been in three novels by this time. I would therefore argue that it was the writing of The Body in the Library and The Moving Finger, as well as the four short stories Strange Jest, Tape-Measure Murder, The Case of the Caretaker and The Case of the Perfect Maid, all published between 1942 and 1944 that told Christie that Marple was going to be a important, recurring character, and thus deserving of a final novel.
Add also that Sleeping Murder arguably doesn't feature the strongest of Christie's mystery plots, and suddenly it seems rather doubtful that the novel as we now know it was completely written during WWII, when her powers of mystery plotting and characterisation were still extremely strong.
It therefore seems to me that while the novel was probably commenced in the early 40s, it was still being re-written and re-drafted for several years after that, at least throughout the entire 40s. There's some corroboration as well to be taken from the fact that Christie continued publishing well above one novel a year from 1946 to 1956 (sixteen novels in eleven years, including three by Mary Westmacott and one autobiographical travel account), as well as the odd short story, and yet there was nothing new published in 1947, just a short story collection with a single new story, The Labours of Hercules. Could that be because around that time, she was heavily involved in re-writing the manuscript that would ultimately turn into Sleeping Murder? I think that might quite probably be the explanation.