It is exactly what he said.
Mat 24:21 For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be.
Mat 24:22 And except those days should be shortened, there should no flesh be saved: but for the elect's sake those days shall be shortened.
And if you still can not understand it I can not help you. Maybe you can grasp a demonstration.
Lets say I, Ron the Great (Smile) knows all and sees all on this earth, ahead of time.
I foretell of the Greatest Ever Troubles coming soon, by way of Ivan the Bruiser. I also say I have foreseen that he would cause much, much troubles and many deaths. But I also foresee Billy the Slicer defeating him on the 2000th day of his rule. Now, when Billy kills him on day 2000 did he cut the troubles that Ivan was bringing on mankind short? Yes, indeed, BUT If I had also foretold this man would rule for 2000 days then he SIMPLY FULFILLED my prophetic foretelling of said events, he DID NOT CUT THE 2000 DAYS SHORT, even though Billy killed Ivan and stopped the troubles he was causing to mankind.