Okay, I established yesterday that you need to find a pre-existing space ship because you can only travel back to the time it was built. A naturally occurring wormhole would work well with both its "mouths" in our galaxy with one effected by a neutron star for example, that would cause time dilation by making one mouth younger than the other. Then you could travel back in time.
Must now consider the time slip: That is how much time will pass on each trip through the wormhole, considering you can figure a way to keep it stable. More tomorrow!