You can't go and kill the guy at a point where you know he has events in his yet. (A person's "yet" is what is known of their personal future). You have to attack him at a point where you he doesn't have any events in his yet that you know about. This also means no killing Hitler before April 30th 1945.
Lucky for us my super power is that I’m completely incompetent at remembering dates and events. Even after reading your comment I couldn’t tell you what date you said or have any idea what it is referring to.