The following is part of a write-up at www.gotquestions.org and explains the 70 years in detail.
  (address:  http://www.gotquestions.org/Printer/tribulation-PF.html)

Question: "What is the Tribulation? How do we know the Tribulation will last seven years?"

Answer: The Tribulation is a future 7-year period of time when God will finish His discipline of Israel and finalize His judgment of the unbelieving world. (1 Thessalonians 4:13-18; 1 Corinthians 15:51-53). The Church is saved from the wrath to come (1 Thessalonians 5:9). Throughout Scripture, the Tribulation is referred to by other names such as:

1) The Day of the Lord (Isaiah 2:12; 13:6, 9; Joel 1:15, 2:1, 11, 31, 3:14; 1 Thessalonians 5:2)
2) trouble or tribulation (Deuteronomy 4:30; Zephaniah 1:15)
3) Great Tribulation which refers to the most intense second half of the 7-year period (Matthew 24:21)
4) time or day of trouble (Daniel 12:1; Zephaniah 1:15)
5) time of Jacob's trouble (Jeremiah 30:7)

 To understand the tribulation period better, it is necessary to understand what was meant by the "70 weeks" spoken of in Daniel 9:24-27. 

 In Daniel 9:24, the Lord says that the "Seventy 'sevens' are decreed for your people and your holy city to finish transgression, to put an end to sin, to atone for wickedness, to bring in everlasting righteousness, to seal up vision and prophecy and to anoint the most holy".      "Your people" refers to Daniels people, the Israelites, or the Jewish people of today - the nation Israel.  The "holy city" refers to Jerusalem - in Israel.

So this "70 week" time will be to

• finish transgression
• put an end to sin
• atone for wickedness
• bring in everlasting righteousness
• seal up [or end] vision and prophecy
• anoint the most holy

(again from the web site  www.gotquestions.org (address:  http://www.gotquestions.org/Printer/tribulation-PF.html))

It is important to understand that when "70 weeks" is mentioned, it is not speaking of a week as we know it (7 days). The Hebrew word (heptad) translated as week in Daniel 9:24-27 literally means "7" and 70 weeks literally means 70 sevens (70 times 7). This time period of which God speaks is actually 70 sevens of years, or 490 years. This is confirmed by another part of this passage in Daniel. In verses 25 and 26, Daniel is told that the Messiah will be cut off "7 weeks and 62 weeks" (69 weeks total) beginning with the decree to rebuild Jerusalem. In other words, 69 sevens of years (483 years) after the decree to rebuild Jerusalem, the Messiah will be cutoff. Biblical historians confirm that 483 years passed from the time of the decree to rebuild Jerusalem to the time when Jesus was crucified. Most Christian scholars, regardless of their view of eschatology (future things/events), have the above understanding of Daniel's 70 weeks.

With 483 years having passed from the decree to rebuild Jerusalem to the cutting off of the Messiah, this leaves 1 seven (7 years) to be fulfilled in terms of Daniel 9:24: "to finish the transgression, to make an end of sin, to make atonement for iniquity, to bring in everlasting righteousness, to seal up vision and prophecy, and to anoint the most holy place." This final 7-year period is known as the Tribulation period--it is a time when God finishes judging Israel for their sin.

When will the 7 year tribulation period start?

Although we do not know the year, month, or day it will begin, we do know that it is still in the future.

 And we can know when it has started.  We don't know when it will start, but there is evidence enough to tell us what to look for to see if it has started.

 In Daniel 9:27 Daniel is given a vision of the end times (the 70 "weeks").  He tells about the first 69 "weeks" and then tells a bit about the 70^th "week" - or the last 7 years.

•   Daniel says, "…then he shall confirm a covenant with many for one week; but in the middle of the week he shall bring an end to sacrifice and offering.  And on the wing of abominations shall be one who makes desolate, even until the consummation, which is determined, is poured out on the desolate."

How will we know the 7-year tribulation period has begun?

 When this covenant is confirmed it will mark the beginning of a 7 year period known as the "7-year tribulation period".  This is the last week of years, the 7 years that Daniel was talking about.  In other words, when a 7 year peace treaty is made with Israel, then we will know that the 7-year period has begun. 

 For the first half of the 7 year period, there will be 2 witnesses of the Lord  (Revelation 11:3-13)  who will prophecy for that time.  They will have supernatural powers and will probably be on the news all around the world because they will not be harmed or killed within that time.. Their time of prophecy is listed as 1260 days.  However, at the end of the 1260 days, they will be killed and put on display for 3 1/2 days.  At that time, they will come to life again and go up to heaven.  This seems kind of strange doesn't it?  Almost like a Science Fiction story – except this will be real.  But if you see these 2 people prophesying for 3 1/2 years and then are killed, you are probably in the middle of the tribulation period.   (See Revelation 11:1-12).  Undoubtedly they won't be the only ones prophesying.  The antichrist will have people doing the same thing.

It says that half way through the 7 year period (3 1/2 years into it), that "he will bring an end to sacrifice and offering."  This means that there has to be sacrifices and offerings going on at that time.  So half way through the 7 year period, the temple has to have been built and in operation. So the temple could be built before the 7 year period or built after it started - within the
first 3 1/2 years. 

 The temple they are talking about is located in Jerusalem.  Since the temple was destroyed in 66 A.D., it hasn't been re-built.  However, preparations are being made.  They are training the priests and they are drawing up plans for the temple.  But they haven't started building it yet.

 The "he" they are referring to is the antichrist.  After 3 1/2 years into the 7-year period he will stop the sacrifices in the temple and set himself up as god.   So the sacrifices are already going on at this time.  It doesn't say how long they were going on, however.  (Daniel 9:27)

Summary

 We don't know when the tribulation period will start, however, it appears to be in the not too distant future, by looking at the middle-east situation. BUT, we can know when it has begun.

So how will we know what the "7-year tribulation" period has begun?

• Israel will sign a 7 year peace treaty.  We don't really know who or what country the treaty is made with, but we know that the signing of it will be the beginning of the tribulation period. ((Daniel 9:27) So, initially, there will be peace in the middle-east.
• During the first 3 1/2 years there will be 2 major witnesses (Revelation 11) prophesying and telling about the Lord.  They will be killed in the middle of the 7 year period - so that means there will be 3 1/2 years yet to go.  (Revelation 11:7)
• Three and a half years after the signing of the 7 year peace treaty, the antichrist will take over the temple and stop the sacrifices.  (Daniel 9:27)  The sacrifices will be going on by this time - in other words, the temple will have been re-built.
• It is at this time that the Rapture will occur.  The Rapture occurs at the last trumpet, which begins the 2^nd half of the 7-year tribulation period. (Revelation 11:15-19) When it occurs, all the Christians will be gone (initially), however, many will become Christians during the last 3 1/2 years.

So the Tribulation period is a 7-year period that is still to come.

What is the "Great Tribulation"?

While the "Tribulation" refers to the whole 7 years, the "Great Tribulation" refers to the last 3 1/2 years of the 7 year tribulation period.  It begins with the 7^th (last) trumpet blast in Revelation (Revelation 9:13-16 & 11:15-19) and is, what some call, the "Day of the Lord" when the wrath of God is poured out on mankind.  Thus it is known as the "Great Tribulation"

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