The Tribulation occurs after the Rapture of the Church. We know this from the other names for the Tribulation: "The Time of Jacob's Trouble" (Jer. 30:7), and "Daniel's Seventieth Week" (Dan. 9:27).
And we know this, is from the Book of Revelation. Revelation is divided into three parts: the past, the present, and the future, all from John's perspective (Rev. 1:19). Chapter one describes what John had already seen. Chapters two and three describe what is known as the Church Age. Revelation 4:1 is a symbolic picture of John representing the Church. He is caught up to heaven; the same thing is said of the Church (Jn. 14:1-3; 1 Cor. 15:51-53; 1 Th.4:13-18). From 4.1, unto the Second Coming described in Revelation 19, the Church is not mentioned as being on earth; it isn't even mentioned in chapter nineteen, but we know the Church will return with Christ, because the Word says so; we will be ever with our Lord (1 Th. 4:17)!
"Daniel's Seventieth Week" is divided into two halves by the prince of the people who destroyed Jerusalem (Dan. 9:26-27; Rome, 70 A.D.). Israel will agree to a seven-year peace "treaty" with the prince. They believe he is their long-awaited Messiah, but right in the midst of the seven years (1260 days - Rev. 11:3; 42 months - Rev. 11:2; a time, times, and half a time - Dan. 12:7), he commits the Abomination of Desolation (Dan. 9:27; 11:31; 12:11; Mt. 24:15; Rev. 13:6), Israel's eyes are opened and they head for the hills (Mt. 24:16; Rom. 11:25)! If the Church was still here when the covenant is confirmed, we would not only warn Israel of the identity of the "prince," but would explain Daniel 9:27 to them!
So, everything is timed based upon Daniel 9:27. No one knows when the Rapture will take place, but we do know that it does not apply to the Church; God has not appointed us to wrath (1 Th. 5:9 - the Tribulation is known as "God's wrath" - Isa. 13:6; Zeph. 1:14; Rev. 6:17). And therefore, no one knows when the Tribulation will begin; once it begins, the timing of the Abomination of Desolation and the Second Coming will be set. Now, if Jesus didn't know when the Second Coming would be, and we know it immediately follows the Tribulation, which follows the Rapture, and no one knows how much time passes between the Rapture and the Tribulation, therefore, no one knows when the Rapture will occur!
The important thing is this: Do you believe Jesus Christ deserves to be your Lord?
When He does come for born again believers, will you be one of them?