The chronology I'm familar with (thanks to my recent course in AmerInds) is that the Na-Dene come into America within the last 7,000 - 5,000 years and are a fairly recent migration.

The Wikipedia article agrees with what I was taught about the language centers for this family of languages:
[en.wikipedia.org]

[www.native-languages.org]

Ruhlen ties the language in with an older languge, Ket, but I note he's not a linguist and he's going on "these words mean about the same thing in both languages" -- a research tactic that I find just a tad suspicious (other material I've read from PhD linguists suggest that you can't go on "words sound the same" but you have to go on the structure of how they form phrases and sentences as well. Having said that, I'll add that I know nothing at all about Ket and Na-Dene.)
[news-service.stanford.edu]