How will the days be shortened? Is God going to lessen time than would normally come? That is a part of the explanation. The implication, though, 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, everyone would die. When He stops the event, time in a sense stops—at least as far as this event is concerned.
To whom are the pronouns referring 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." These pronouns refer to those who understand the prophecies and are alive at the time these things are taking place. How many people are involved? It is unspecified.
One thing is clear. There is no doubt that, in this prophecy, deliverance involves flight (at least to those who are around Jerusalem during this unprecedented distress). In this case, to flee in no way implies flying. The verb here is phuego, and it means to flee, to escape danger. It indicates nothing other than escaping by running—shoe-leather express.
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 21-22 make it clear "that no one would be saved alive." He means no one on earth, not just that no one around Jerusalem would be saved alive. This time is so bad that even the elect would die, except for God's intervention. Notice that God, through His servant Jesus Christ, says, "Don't stay in the midst of the trouble—get out."
Considering the timeframe, Psalm 91 would have to be modified to apply it directly to us, because our understanding from other portions of God's Word is that He expects us to get out, to flee to some designated place called "your chamber" or "her place." It involves segregating ourselves away from something. We can also understand that we will have help from God in segregating ourselves, as I Corinthians 10:13 indicates. He makes a way of escape that His people can take, just as He did for Israel. He opened up the Red Sea so that they could escape. They walked to safety, fleeing from the Egyptian army. Nevertheless, God intervened.
