When trying to come up with a comprehensive timeline for the Oz series, such as the Royal Timeline of Oz, the main problem actually comes in with the first few books.
It's a popular idea that the books take place around when they were published, or maybe a year or two before, and for the most part the evidence supports this. With the earlier books, though, it's trickier.
The Wonderful Wizard of Oz was published in 1899 and The Emerald City of Oz in 1910, yet Dorothy is still only ten or eleven at the time of the latter. So it's pretty much necessary to condense the number of years over which these books take place, although exactly how can't be determined for certain. I've seen it suggested that Emerald City could take place as early as 1903, although most chronologists choose a somewhat later date. The earthquake in which Dorothy is caught in Dorothy and the Wizard is almost certainly based on a real one that hit San Francisco in 1906, but we're not specifically told that it IS this quake, and I find it rather unlikely that six years had passed between Wizard and DotWiz.
To further complicate matters, there are several books that take place during the period in which Dorothy was still living in Kansas, probably mostly due to copyright reasons. Most of these stories take place over only a few days, but whether or not someone accepts them can affect how long it was between Dorothy's first visit to Oz and her finally deciding to live there. Tyler Jones proposed that Dorothy's aging could have been slowed by her time in Oz, but I'm not so keen on that idea.
In addition to Dorothy, we also have Button-Bright, who first visits Oz when quite young in Road, and is still younger than Trot when he moves there in Scarecrow. Since Trot claims to be ten in Giant Horse, it's likely that Button-Bright is nine years old at the most. Six years passed in between the publication of these two books, which means it might be helpful to condense the years between them as well, albeit not as much as with the earlier books.
Rinkitink is also an interesting case, as it's based on an earlier manuscript, but with references added to fit it into its designated place in the series. J.L. Bell proposed the idea that the Nome King in Rinkitink might actually be Ruggedo, which would necessitate pushing this one back to before Tik-Tok. This would mean disregarding King Rinkitink's song about Hank the Mule, as Hank doesn't come to the Emerald City until the end of Tik-Tok, but that's hardly a significant part of the story. Still, I generally prefer keeping Rinkitink as the tenth book in the series, and just figuring there was some other reason why Kaliko was acting out of character.
As far as the Ruth Plumly Thompson books go, 1930's Yellow Knight has Speedy thinking of Charles Lindbergh, who made his famous flight in May 1927. As such, the story probably doesn't take place any earlier than 1928. I usually like to think of her books as taking place the year before they were published, but there could be a problem here as well.
Dorothy Maryott's essay on Silver Princess says that Thompson began writing it in February 1937, and the story takes place in May of an unspecified year. So wouldn't it have had to have taken place in 1936 at the latest, in order for it to be history when Thompson wrote it? I think there's plenty of wiggle room here, though. Sure, the authors presented themselves as historians, but this can't be taken TOO seriously. Some more recent authors, particularly Chris Dulabone, have taken to actually writing the year in which a book takes place within the story, which is helpful to those of us who want to keep timelines straight, but comes across as a bit weird when the characters themselves use the AD system of years. In that way, I prefer Melody Grandy's books, which tend to date everything from the beginning of Ozma's reign. There's no specific date for this that's agreed upon either, but 1902 is a popular supposition.