Here's a complete video guide by MrRoflWaffles, all credit goes to him.
https://www.youtube.com/watch?feature=youtu.be&v=v3v_IKf... There is a little mistake in this video, that is when you travel back in time for the second time. Instead of going to the box, go to the book on the right side of the room (when the teleporter is behind you) and hold
. Now the chest will open and you will be able to grab the Keeper slab.
Good luck!