Now how is it going to be shortened? Is God going to lessen time than would normally come? I think that is also a part of the explanation. I think the inference, though (right within the context), is that God will stop short what is occurring—lest everybody be killed. If He allowed the events that were taking place to continue, everybody would die. So, when He stops the event, time (in a sense) stops right there—at least as far as this event is concerned.
Now to whom are the pronouns referring to here? Verse 15, "Therefore when you." Verse 16, "Then let those." Verse 17, "Let him."Verse 18, "Let him." Verse 19, "But woe to you." And on and on it goes. The pronouns refer to those who understand the prophecies and are alive at the time these things are taking place. Now how many people are involved here? Well, it is unspecified.
One thing is clear. There is no doubt that, in this prophecy, deliverance involves a flight (fleeing—at least to those who are around Jerusalem during its unprecedented distress). Now in this case, to flee in no way implies flying. The verb here is pheugo and it means to flee, it means to escape danger. There is no indication of anything other than escaping by running—shoe-leather express.
Now the context of the chapter is literal and physical. It is not figurative and spiritual; it involves physical survival worldwide, though the prophecy itself focuses on Jerusalem. It is worldwide because verses 21 and 22 make it very clear "that no one would be saved alive." He means worldwide—not just that nobody would be saved alive in Jerusalem. We are talking about a worldwide occurrence, so it involves physical survival worldwide. It is so bad that even the elect would die, except for God's intervention. Now I want you to notice—this may not seem all that important, but it is important—that God, through His servant Jesus Christ, says "don't stay in the midst of the trouble. Get out."
