How do we Know the time of the Tribulation is 7 Years? This is known by Doing the math in Daniel 9 of the 70 weeks of years

Daniel 9:24-27 “Seventy weeks are determined For your people and for your holy city, To finish the transgression, To make an end of sins, To make reconciliation for iniquity, To bring in everlasting righteousness, To seal up vision and prophecy, And to anoint the Most Holy. “Know therefore and understand, That from the going forth of the command To restore and build Jerusalem Until Messiah the Prince, There shall be seven weeks and sixty-two weeks; The street shall be built again, and the wall, Even in troublesome times.“And after the sixty-two weeks Messiah shall be cut off, but not for Himself; And the people of the prince who is to come Shall destroy the city and the sanctuary. The end of it shall be with a flood, And till the end of the war desolations are determined. 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.”
A) Notice what are the 70 weeks of years are for :
1) To finish the transgression
2) To make an end of sins
3) To make reconciliation for iniquity
4) To bring in everlasting righteousness
5) To seal up vision and prophecy
6) And to anoint the Most Holy
70 weeks of years is 490 years 70 × 7 =490
B) “Know therefore and understand, that from the going forth of the command to restore and build Jerusalem Until Messiah the Prince, there shall be seven weeks and sixty-two weeks. Even in troublesome times.
7×7= 49
62 × 7= 434
49+434 =483
Persian king Artaxerxes on March 5, 457 BC according to archeology commanded To restore and build Jerusalem.
Messiah was born between 6 BC Herod died in 4 BC & he sent to kill the children who was 2 years or younger in his attempt to kill Jesus.
Matthew 2:16 Then Herod, when he saw that he was deceived by the wise men, was exceedingly angry; and he sent forth and put to death all the male children who were in Bethlehem and in all its districts from two years old and under, according to the time which he had determined from the wise men.
C) "And after the sixty- two weeks Messiah shall be cut off, but not for Himself; And the people of the prince who is to come Shall destroy the city and the sanctuary.
Death of the Messiah Jesus was in 26 AD Jesus started his ministry at age 30 Luke 3:23 Now Jesus Himself began His ministry at about thirty years of age... And lasted till his death about 3 years this is assumed by counting the feast Passover twice in the gospel of John and assuming the unnamed feast to be Passover as well. Romans destroyed the Temple by the command of Titus was in 70 AD
D) The end of it shall be with a flood, and till the end of the war desolations are determined. 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.
457 BC to 26 AD is 483 years this leaves 7 years left of the Tribulation which Antichrist will make a 7-year covenant and break it in the midst.