One of the reasons I want to learn Astral Projection is to travel back in time. I believe I read that it is possible to do this.
I just think it would be very cool to visit my childhood again, or witness a historical event, or watch babe Ruth hit a homerun.
I have never done it before but yes, in the Astral Planes it is possible to travel back in time. The Etheric Planes have more of a connection to the RTZ so I don't think it's possible there. All you have to do is 'will' yourself to a time that you want to go, just like willing yourself to a location. Good luck! :-)
I agree with Magickist.