We shouldn't be challenging what Obama claims he said on Sept 12th - We should be accepting that he called it an "act of terror" on Sept 12th, and then challenging him to defend the two weeks that followed - challenge the fact that he and his administration, knowing that this was in fact a terrorist attack, chose to spend two weeks denouncing the video instead of denouncing the terrorism:
See the link for an effective example of this:
Obama's Benghazi Lies