This is Olson's litmus test for the all-too-often knee-jerk answer that God can do anything.  If God can't change the past, then God can't do everything or anything.  Olson posits that the Bible does not give examples of God changing the past, nor does the Bible encourage us to pray for God to change the past.  Add to that the complexities posed by the concept of someone traveling back in time and changing something, and it seems clear to Olson (and apparently other scholars) that God cannot change the past (page 24, Questions to All Your Answers)

Olson really ignores the classical response or qualification of the assertion that God can do anything.  The classical qualification is that God cannot do anything against His nature.  In other words, as a perfectly loving and good being, He can't do anything evil.  He can't act contrary to His essence.  Olson implies this on page 25, but never comes out and says it.

Rather, he makes a big deal about how irrational and illogical it is to think that God can change the past.  Once again, this demonstrates a good limitation on the arena of reason and the intellect.  True, in popular science fiction as well as scientific theory, it is posited that any change made to the past by someone traveling to the past would result in cascading changes of an unforeseen nature.  I think of Ray Bradbury's classic short story A Sound of Thunder about just such a situation (http://www.lasalle.edu/~didio/courses/hon462/hon462_assets/sound_of_thunder.htm).  

These theories may be accurate, but they also presume that these changes would be detectable in some way - at the very least, detectable to the person(s) returning from a visit to the past.  Is this necessarily the case?  

Some Christians talk about the redeeming and reconciling act of Jesus' death and resurrection, and the idea of the effects of that atoning event rippling out into time.  Are the ripples only extending into the future, the future we now live in and our children and grandchildren will live in?  Do we know that those ripples don't extend equally into the past in ways that we can't understand and can't detect?

Olson appears to say that because of the complexity involved, God can't change the past.  But if God created the past - as well as the present and the future - how can we reasonably say that the past is fixed and immovable?  It's a small point, but an example of assuming that because we think we've thought out how things ought to be, that God is therefore limited in some respect.