Theres this idea i had if you go into a time machine and reverse time you are no longer standing in the place you were when time reverses, your in the time machine, so particals that hit have your body would pass right through colliding in some other way, this would lead to what i call the reverse butterfly effect, and the farther you go back the more drastically different the past becomes.
Conclusion by going to the past you change the past before you ever step foot in the past.