Board Thread:The Flash discussions/@comment-26855145-20161005152403/@comment-25449165-20161006032546

The Flash and the Reverse-Flash that were present during the very original timeline when Nora was killed, were from the distant future, where I would assume would be the date that the first Harrison Wells we were introduced to was from. That event would begin the story of Barry Allen (the one we know from s1) and he would go through events that lead to the Barry Allen, or the Flash that went back in time with the RF to the point where Nora was killed. (Timeloop here). He would eventually propel the Barry Allen of that time into becoming the Barry Allen he is now.

Again, it's complicated. But it's all Greg Berlanti's ideas. If you really want some concrete answers, you'll have to hear it from him directly.