The whole seven years is the Tribulation period, the time of Jacob's trouble (Jeremiah 30:7). Jesus and John (in Revelation) both refer to the last 3 1/2 years as the Great Tribulation. All seven years are the wrath of God, not just the last half; therefore the church must be raptured out before all seven years, not just at the midpoint. If you compare Isaiah 13 and Luke 21, you will see the descriptions are the same - and Isaiah makes it very clear that this time is a time of God's wrath, where He is pouring out His indignation.