Hyborian Age timeline

[The King]

It's not really important to give an exact dating of the Hyborian Age in comparison with real history. However essays like those of Dale give us very good insights of the Hyborian history (with its migration of people, the reasons of its ruins, etc.) that we don't have in many settings.
Studying the Hyborian Age you will know why Stygians behave as they do and would probably be never be friends to the folks of Khitai.
Then if you study the Thurian and the Hyborian Age you know why Picts and Cimmerians are hereditary foes.
Howard gave much coherence to his world and it is not always easy to understand all the phases of civilization. I think Dale do a great favor to us, roleplayers, to understand better between the lines and the many sorties Howard wrote.
Then Connexion to real history isn't important because in his introduction to the Hyborian Age (not printed in the Core book), Howard writes :

Nothing in this article is to be considered as an attempt to advance any theory in opposition to accepted history. It is simply a fictonal background for a series of fiction-stories. When I began writing the Conan stories a few years ago, I prepared this "history" of his age and the peoples of that age, in order to lend him and his sagas a greater aspect of realness. And I found that by adhering to the "facts" and spirit of that history, in writing the stories, it was easier to visualize (and therefore to present) him as a real flesh-and-blood characterrather than a ready-made product. In writing about him and his adventures in the various kingdoms of his Age, I have never violated the "facts" or spirit of the "history" here set down, but have followed the lies of that history as closely as the writer of actual historical-fiction follows the lines of actual history. I have used the "history" as a guide in all the stories in this series that I have written.

In other words, Howard invented a world much like Tolkien.

 

[Faraer]

In terms of literary morphology, Howard's prehistory is somewhere between a Tolkienesque or Smithesque secondary world and historical fiction.

I haven't read much of Howard's and Lovecraft's correspondence, but there's the reference in "The Shadow Out of Time" to "Crom-Ya, a Cimmerian chieftain of 15,000 B.C.", which will have influenced the dating. See how the Hyborian Age is dated in Cthulhu Mythos timelines, such as Shannon Appel's in Encyclopedia Cthulhuiana. Would Howard have accepted this reference, or was the link between their mythoi looser than that? We may wonder what Hyborian Age followers Cthulhu, Gol-goroth, Kathulos, Koth, Tsathoggua, Yog-Sothoth, and Yuggoth (glossed as 'the Nameless Old Ones' in PS) had.

 

[The King]

In terms of literary morphology, Howard's prehistory is somewhere between a Tolkienesque or Smithesque secondary world and historical fiction.

I haven't read much of Howard's and Lovecraft's correspondence, but there's the reference in "The Shadow Out of Time" to "Crom-Ya, a Cimmerian chieftain of 15,000 B.C.", which will have influenced the dating. See how the Hyborian Age is dated in Cthulhu Mythos timelines, such as Shannon Appel's in Encyclopedia Cthulhuiana. Would Howard have accepted this reference, or was the link between their mythoi looser than that? We may wonder what Hyborian Age followers Cthulhu, Gol-goroth, Kathulos, Koth, Tsathoggua, Yog-Sothoth, and Yuggoth (glossed as 'the Nameless Old Ones' in PS) had.

I know that they liked to borrow some parts of the myths. For instance HPL used Howard as a character in one of his stories (in which of course the hero turns insane) and I think Howard did the same in one of his horror tales.
But they shared something of a common origin of their myths even if I'am not sure that the Howardian god Yog is the same as Yog-Sothoth (de Camp probably established this relationship). For sure, there are creatures from the outer void (reference to space) as in the Women of the lost valley as well as from other "spheres" (reference to other planes as Thulsa Doom mentions in one of the Kull stories.

 

[Faraer]

I don't think Yog has to be Yog-Sothoth (Yogah certainly isn't), but the entities above are all named in REH stories.

Quick notes on "Brachan the Kelt", a lesser-known James Allison reincarnation story, which I read yesterday. It matches the Hyborian continuity in most respects -- after the 'terrific convulsion of the earth, carving out the lands as they are known to moderns', Celts settled on the western shore of the long inland sea -- but they're blond, and don't have steelworking.

The name 'Brachan' is a good Cimmerian one. The other two named characters are Taramis (same name as used in "A Witch Shall Be Born") and Jogah.

'Lions roamed through what now is Europe, gigantic brutes, huger and fiercer than any today existent. There were cave bears, sabre-toothed tigers, giant buffalo and elk, panthers...'

'...I asked for her -- asked? A Kelt did not _ask_ for anything, save from his own chief. I demanded...'

[edit]
Also, 'Yet we bore weapons and implements of bronze, and understood the art of weaving rushes into baskets, and of manufacturing flax into cloth.' Since the Kelts are less technologically advanced than the Hyborians, this suggests the Hyborians had cotton (which I believe is not evident from the Conan stories, though they mention spun fabrics). De Camp in "Hyborian Technology" (The Blade of Conan) says 'The Hyborians presumably used wool and possibly linen.'

 

[Deathdealer]

For instance HPL used Howard as a character in one of his stories (in which of course the hero turns insane) and I think Howard did the same in one of his horror tales.

that was Robert Bloch, not Howard. Still, there are definite elements of Lovecraft's Mythos in Howard' work.