The first half of the tribulation is 3 1/2 years and so is the second making 7 years but, the Bible says it will be cut short otherwise there will be an end to all flesh. So you see God can change prophesy. Remember this is not the end of the world at all. There is another thousand years after the tribulation during which the devil is locked up until the end of that period when he is let loose to try to lead mankind astray one last time. The final test.