"Later days" was a term used like "dude." Depending on the inflection in your voice, it could mean several things.
It could mean everything from "i"ll see you tomorrow," to "if I ever see you again, I will kill you."
In SoCal, the term eventually shortened down to "later on."
This became a generic term for, "see you later." It lost most of its inflection power at this point.
Probably a year later or so, (the time it takes for slang to morph) the term shortened down to "later."
It lost all inflection and became a simple goodbye.