God created the heavens and the earth in six days and then rested on the seventh day, creating an example for us to follow. Accordingly, Jewish and Christian scholars both teach that history is divided into seven one-thousand-year periods called millennia, and the seventh millennium will be a period of peace and tranquility. During the seventh millennium, Jesus will reign as king of the world, and His followers will reign with Him. The exciting news is that only a few years remain before the end of the sixth millennium, but this is a deadline. We must first help every ethnic nation hear the Gospel of the kingdom of Jesus. Furthermore, our final efforts to do this will determine our standard of living and responsibilities during the seventh millennium.

The idea of 7000 years of history is nothing new. Jewish scholars taught a 1000-year Shabbat after 6000 years, and early Messianic (aka Christian) leaders taught it in more detail. Some people call it the Millennial Day Theory. What is significant is that we may be very close to the end of the first six millenia. This article is not setting dates, but if the theory is true, then we can expect the final 42 months of the Great Tribulation to start soon.
Seven millennia of history was originally a Jewish idea. Although most Jews do not recognize Jesus (Yeshua) as their Messiah, their oral teachings taught the concept of seven millennial days. Eventually, Jewish scholars systematically recorded their oral teachings in writing. Here are two quotes from the Babylonian Talmud which may have formed a basis for the Millennial Day Theory:
Rav Ketina says: Six thousand years is the duration of the world, and it is in ruins for one thousand years. The duration of the period during which the world is in ruins is derived from a verse, as it is stated: "And the Lord alone shall be exalted on that day" (Isaiah 2:11), and the day of God lasts one thousand years. Abaye says: It is in ruins for two thousand years, as it is stated: "After two days He will revive us; in the third day He will revive us, and we shall live in His presence" (Hosea 6:2). - Sanhedrin 97a, The William Davidson Talmud
It is taught in a baraita in accordance with the opinion of Rav Ketina: Just as the Sabbatical Year abrogates debts once in seven years, so too, the world abrogates its typical existence for one thousand years in every seven thousand years, as it is stated: "And the Lord alone shall be exalted on that day," and it states: "A psalm, a song for the Shabbat day" (Psalms 92:1), meaning a day, i.e., one thousand years, that is entirely Shabbat. And it says in explanation of the equation between one day and one thousand years: "For a thousand years in Your eyes are but like yesterday when it is past, and like a watch in the night" (Psalms 90:4). - Sanhedrin 97a, The William Davidson Talmud
When Messianic communities (aka churches) developed, believing that Jesus (Yeshua) is the promised Messiah, they continued to use the Jewish Scripture (aka the Old Testament) for doctrinal authority. (And we should too!) Jesus merely consummated it (Romans 10:4) and taught it in all its fulness (Matthew 5:17,18). (And under the inspiration of the Holy Spirit, the New Testament writers recorded it and further clarified it for the benefit of all peoples.) In fact, the majority of Messianic (aka Christian) leaders at the beginning were Jews, so they relied on the traditional thinking of Jewish scribes as long as it didn't contradict the teachings of Jesus. Then, the Spirit of God gave them further insight. For example, the Apostle Peter wrote,
But, beloved, do not forget this one thing, that with the Lord one day is as a thousand years, and a thousand years as one day - 2 Peter 3:8 NKJV
The Millennial Day Theory is a very simple summary of world history, dividing it into seven 1000-year periods based on Biblical genealogies. It was taught by some early Church leaders, like Papias and Tertullian, and it is based on the idea of Shabbat (sabbath) - a day of rest. Just as God rested from creation on the 7th day (Genesis 2:2,3), all people should rest one day per week. Also, after six millenia, the earth will rest, as Jesus Christ rules for 1000 years.
Then I saw an angel coming down from heaven, having the key to the bottomless pit and a great chain in his hand. He laid hold of the dragon, that serpent of old, who is the Devil and Satan, and bound him for a thousand years; and he cast him into the bottomless pit, and shut him up, and set a seal on him, so that he should deceive the nations no more till the thousand years were finished. But after these things he must be released for a little while. And I saw thrones, and they sat on them, and judgment was committed to them. Then I saw the souls of those who had been beheaded for their witness to Jesus and for the word of God, who had not worshiped the beast or his image, and had not received his mark on their foreheads or on their hands. And they lived and reigned with Christ for a thousand years. But the rest of the dead did not live again until the thousand years were finished. This is the first resurrection. Blessed and holy is he who has part in the first resurrection. Over such the second death has no power, but they shall be priests of God and of Christ, and shall reign with Him a thousand years. - Revelation 20:1-6 NKJV

The Bible does not specify exactly how long the Great Tribulation lasts. It is mentioned in Daniel 12:1, Matthew 24:21, Revelation 2:22 and 7:14, but the time period is not explicitly stated. God left this matter ambiguous in His Word. We may already be in it! However we do know that there will be a final period of 3.5 years, 42 months or 1260 days which will be the most difficult, based on Daniel 7:25, 12:7 and Revelation 11:2,3,6,14 and 13:5 (seven occurrences). A careful literal reading of these verses shows that this is in the near future:
It is important to understand that the primary purpose of the Great Tribulation is for God's people (of all ethnic backgrounds) is to seek first the Kingdom of God and His righteousness (Matthew 6:33) as they participate in the Great Commission (Mark 16:15, Matthew 28:19). The Lord Jesus wants people from every people group to experience His fulness (John 1:16), so His design is that a full number of Gentiles come into His Kingdom so that His Bride has all the good cultural traits of every culture (Romans 11:25; Ephesians 2:11-3:11). In other words, during the final 3.5 years, God's people need to complete the Great Commission and prepare for the Wedding of the Lamb. After all, if the Lord Jesus were to return today, He would be unequally yoked in His relationship with the Church. God is shaking the Church to make this happen (Hebrews 12:26-29), and this shaking or judgment begins with God's people (1 Peter 4:17).

We must all be exposed before Christ's Judgment Seat, and rewards will be distributed based on faithfulness to opportunity. Rewards will include our standard of living during the seventh millennium:
If you faint in the day of adversity, your strength is small. Rescue those who are drawn toward death, and hold back those stumbling to the slaughter. If you say, "Surely we did not know this," does not He who weighs the hearts consider it? He who keeps your soul, does He not know it? And will He not repay each man according to his deeds? - Proverbs 24:10-12
"And that servant who knew his master's will, and did not prepare himself or do according to his will, shall be beaten with many stripes. But he who did not know, yet committed things deserving of stripes, shall be beaten with few. For everyone to whom much is given, from him much will be required; and to whom much has been committed, of him they will ask the more." - Luke 12:47,48 NKJV
For we must all appear before the judgment seat of Christ, that each one may receive the things done in the body, according to what he has done, whether good or bad. Knowing, therefore, the terror of the Lord, we persuade men; but we are well known to God, and I also trust are well known in your consciences. - 2 Corinthians 5:10,11
The GOOD NEWS is that the Lord of the Vineyard is very eager to have His harvest completed, so He is willing to put unskilled workers to work at the eleventh hour:
"For the kingdom of heaven is like a landowner who went out early in the morning to hire laborers for his vineyard. Now when he had agreed with the laborers for a denarius a day, he sent them into his vineyard ... And about the eleventh hour he went out and found others standing idle, and said to them, 'Why have you been standing here idle all day?' They said to him, 'Because no one hired us.' He said to them, 'You also go into the vineyard, and whatever is right you will receive.' So when evening had come, the owner of the vineyard said to his steward, 'Call the laborers and give them their wages, beginning with the last to the first.' And when those came who were hired about the eleventh hour, they each received a denarius." - Matthew 20:1-10
In conclusion, we should all be prepared for accelerating rates of chaos, rebellion, lawlessness, fearful events, global government and a control grid to support the global government. We We may already be living in the Great Tribulation. At the same time, in the midst of tribulation and crisis, there will be great opportunities to preach the Gospel to people without an eternal hope, and in this process the Lord Jesus' bride will be purified before their wedding. The key is to be seeking first His Kingdom and His righteousness (Matthew 6:33). If we choose to participate in the completion of the final harvest, God will reward us, and it may be more than we expect.

For man also knows not his time: as the fishes that are taken in an evil net, and as the birds that are caught in the snare; so are the sons of men snared in an evil time, when it falls suddenly upon them. - Ecclesiastes 9:12