Lilo and Stitch is modern. Most of the others aren't.
Also I think Atlantis is specified as 1914. Most settings they used are before the 20th century.
edited 16th Jan '15 9:45:23 AM by AHI-3000
Great, you found a couple of exceptions. I can think of more. That doesn't change the fact that most Disney movies cannot be confidently located in a single year or even a single decade. They're timeless, and that's fine. But it does mean that I can't see time travel as the most appropriate way to connect different parts of the Disney universe.
That's the great thing about Happily ever after: John smith is allowed to visit and the cheapquels don't exist. They also meet each other regularly in the house of mouse. Located in Toontown.
Toontown could be a sort of central nexus where the boundaries come down and the different realms interact without it being weird.
Still just awesome like that
Lilo And Stitch I can see being an earlier era as far as the not-so-distant past, like maybe the '70s or '80s. (It'd have to at least be an era with home video, considering how Lilo talks about watching Blue Hawaii, and at least post-1960 considering all the Elvis Presley music.)
Insert witty 'n clever quip here.
Darkwing Duck pointed at the possibility of there being an actual Disney Multiverse, including a parallel to DC's Earth Prime where Disney characters were Animated Actors. The comics especially take that ball and run madly with it.
Time travel is what I thought of first as well, especially since most of the movies can be pinned to a few general time periods. As pointed out, they're also not strangers to screwing around with time frames or locations of things if they feel the settings are congruous enough in the first place. Add in the possibility of some characters being actually time/universally displaced as part of the story and you're golden.
Having the story follow a single group of characters having a series of adventures would remove the issues almost entirely, since unlike House Of Mouse they would have no need to portray every single Disney character existing in the same setting at the same time. IMO, the main issue would be finding a focus character - Mickey is obvious, but I like the idea of a loose group of protagonists composed of various Disney characters.
edited 16th Jan '15 2:43:29 PM by KnownUnknown
"The difference between reality and fiction is that fiction has to make sense." - Tom Clancy, paraphrasing Mark Twain.
That was the intention of The Search for Mickey Mouse. But of course, that was never even produced.
I keep imagining the heroes being Scrooge, Donald and the triplets, but then that's because I keep imagining this kind of adventure being written in the vein of Don Rosa. 
I recall the pitch for Search involved Minnie, Peter Pan, and Detective Basil at least.
That's another thing: Basil of Baker Street would be an awesome idea for a tv series, especially now that they have a healthy dose of new Sherlock Holmes media to homage (in the same way reviving Darkwing Duck would be interesting now given how standard superhero stories just ripe for parodying have become in film these days).
edited 16th Jan '15 9:05:34 PM by KnownUnknown
"The difference between reality and fiction is that fiction has to make sense." - Tom Clancy, paraphrasing Mark Twain.
And it would be animated by Mercury Films! FUND IT!
I don't think most of the movies are pinned to specific enough times for that to make much sense.