Actually, the great tribulation begins from when the abomination is set up:
"So when you see standing in the holy place ‘the abomination that causes desolation, spoken of through the prophet Daniel—let the reader understand—then let those who are in Judea flee to the mountains.Let no one on the housetop go down to take anything out of the house.Let no one in the field go back to get their cloak.How dreadful it will be in those days for pregnant women and nursing mothers!Pray that your flight will not take place in winter or on the Sabbath.For then there will be great distress, unequaled from the beginning of the world until now—and never to be equaled again."
The great tribulation begins with Jesus saying "So when you see standing in the holy place ‘the abomination that causes desolation" which is directly linked with the Lord's warning to flee out into the mountains and it being the greatest time of tribulation since mankind has been upon the earth. The great tribulation ends in Matt.24:29-31 with Christ returning to the earth to end the age and establish his millennial kingdom.
In addition to the above, we have a reference to that time of great tribulation for the great tribulation saints referred to in Dan.7:25 as a time, times and half a time, (3 1/2 years) with that same 3 1/2 years mentioned in Rev.13:6-8, when it is said that the beast is given authority to rule for 42 months, where during that time he wages war against the saints and conquers them.
"He will speak against the Most High and oppress his holy people and try to change the set times and the laws. The holy people will be delivered into his hands for a time, times and half a time. - Dan.7:25
"The beast was given a mouth to utter proud words and blasphemies and to exercise his authority for forty-two months. He opened his mouth to blaspheme God, and to slander his name and his dwelling place and those who live in heaven. He was given power to wage war against the saints and to conquer them. And it was given authority over every tribe, people, language and nation. - Rev.13:6-8
The day of the Lord, also referred to as "the hour of trial," is neither a day nor an hour in length. The day of the Lord begins with the gathering of the church and continues throughout the entire seven years.
the above is just referring to what happens at the end of that 3 1/2 years, i.e. immediately after that 3 1/2 years of great tribulation, with Christ returning in conjunction with the events of the sun, moon and the stars.