"Tell them I am failing. And that he is stronger. Tell them that security cannot be guaranteed. Tell them that there is little time. And that I am crumbling."
One of AR's messages, above, states that security cannot be guaranteed. This sounds incredibly like a more formal to say the classic "safety not guaranteed."
There could be, but the majority of the people (now) like to believe that no time travel has occurred with AR. That he is just booting up in our time, and has yet to experience the events of the Halo trilogy.
Also, if the Forerunner could time travel at will, I'd bet they'd go back in time long enough to prevent the rise of the Flood.