Manhattan's understanding of time changes the very concept of time. According to Manhattan, time is not linear, but rather we exist in all moments simultaneously.
EG: at this time today I am in a diner having a fish sandwich. However right now in 1991 I am at a Burger King across town with my high school friends having a whopper with cheese and onion rings.
Right now on April 19, 1973 I am being born....
Essentially every second of our life occurs on different planes simultaneously.
If his concept is correct, then time alteration would be difficult.
As for a Q. They were shown to have the ability to alter time, but they didn't always have the foresight to see what was coming. Though I know in one of the books Q was present at the same event three times, and he could witness his past selves there. Though his past selves didn't seem to have any knowledge of their future selves.
Dr. Manhattan! Why do people keep trying to match this guy up with people?! He is literally unbeatable. Period.