I assume you're talking about his aside in Sign of the Moon.
I don't think it was really "time travel", in the strict sense of physically going back in time and experiencing events in that era/time period. As
Kibui stated above, Jayfeather is the reincarnation of a cat from that time period, so it's more like a very strange, lucid dream that ties in (obviously) to his power.
If he is in fact reliving things that Jay's Wing did at that point, it begs the question of how
he (being Jayfeather) knew how to act and say. I guess it's a bit like Cinderheart, who was able to tap into Cinderpelt's memories through dreams.
Not to get into paradoxes or anything, but I suppose you could say that neither Jayfeather nor Jay's Wing exist in the Ancients' time period, as Jayfeather is the one controlling what Jay's Wing says or does, but Jayfeather really doesn't know how Jay's Wing acts or talks. Jayfeather is technically the reincarnation of Jay's Wing, but it's really more the opposite, as the only "Jay's Wing" we know is the one Jayfeather is temporarily replacing during his lucid dream thing.
Blech. The whole saga is poorly written, thought-out, and presented. It's a case of "show, don't tell", as we never actually get to see the "real" Jay's Wing- just the one Jayfeather replaces. This is why an arc or SE about the Ancients would actually be interesting.
