....Which is why this is an alternate, more interesting 1800s.  My only problem is that is it possible for me to just imply that this is happening a long time ago with out some giant header that's like "1821: London"?  Can I just have people on horses and crap like that and you'll just suppose this happened a long time ago?  And, of course, by characters dialogue?  

I have no clue how to write historically accurate.  Help.

The color and flavor of the writing will give us an idea of when it occurred as well as the costume design and other stuff we don't have much to do with. There is no requirement to put a date on anything. The only reason you would do so is if that date were important for some reason.

You just want to imply the time, then imply it. No problem there. Write what you see.