The Sixties really didn't ever end.
The Roaring Twenties are still around.
There’s a place where you can go and see Elvis any day of the week.
There are cracks in the world, you see, where you can slip through and fall into not-quite-the-past. You’re not going to fall into 1923 if you slip into that old speakeasy through the fourth window on the left on a Tuesday afternoon, but you will find yourself in a version of the Twenties as it was, as it is thought to be now, and as its inhabitants thought it was. Some, like men asleep, go through eternal lives in eternal times. Nothing happens to them that has any lasting significance. They neither age nor notice when the clock starts over and March 5, 1929 becomes August 2, 1923. Others are aware of the nature of their world, which is an ability that few of them find to be worth bragging about. They notice everything that the rest of the world does not, but after their minds are awakened the passing of the years leaves a mark on them and if violence, sickness, or accident do not fell them then age surely will. Even worse, they are still no more capable of effecting any kind of permanent change in their world. As soon as they become fully conscious the Awake are doomed to watching the same play over and over until they die and are utterly forgotten by everyone except for the other Awake.
In order to achieve anything the Awake must work through people from the "progressing" world. They cannot leave their own world, which makes it difficult, but as soon as someone on our side of the divide finds her way to their world and makes contact with one of the Awake then it is possible to use her to recruit others. You can't alter history by going back to these faux-pasts. What you can do is alter how it was perceived, which can be just as useful if you’re savvy enough to figure out how to do it, and you can also bring things back, although you've got to be careful that you don’t disrupt things too much. "Progressing" people do not age when they are in these faux-pasts but when they return to our world the years rapidly catch up to them, generally at a rate of one year every month.
These worlds are connected not only to our world but also to each other. The doorways into the earliest time periods are most often long destroyed but they might still exist in other faux-pasts.
There's a wrinkle to everything, though, and these worlds are no exception. In them there is something called the Green, a force that exists in the space between seconds and the dead past. The Green seems to patrol these places against intruders and interlopers (the Awake are ignored because they can't make permanent changes) and appears in forms amenable to the world around it, as a dragon, gangsters, Nazis, a Soviet spy, wolves, and more. Anything that its targets will perceive as malevolent and antagonistic. No matter what, however, the color green is present in some way, whether it be in the Green's scales, cuff links, glasses, eyes, tie, or something else.