Revelation 12:6 The woman, Yisra’el, who also represents the assembly of today, will be hidden in the wilderness for one thousand two hundred and sixty days (1260 divided by 360 = 3 1/2 years).
NOTE: There is an extra 30 days here but no one has yet figured out what it is.
The way I understand it these people that believe this is “The Great Tribulation” and that is in two segments of 3½ years each, get it from Daniy’el 9:24-27 (below).
Daniel 9:24-27 . . seventy weeks are determined for your people and for your sacred city to finish the transgression, to make an end of sins, to make an atonement for iniquity, to bring in everlasting righteousness, to seal up vision and prophecy, and to anoint the Most Sacred. 25Know therefore and understand that from the going forth of the command to restore and build Yeruwshalom until the anointed Prince, there shall be seven weeks and sixty-two weeks. The street shall be built again, and the wall, even in times of anguish. 26 And after the sixty-two weeks Mashiach shall be cut off, but not for Himself, and the people of the prince who is to come shall destroy the city and the sacred place. The end of it shall be with a flood and desolation is decreed until the end of the war. 27 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 abominable things He shall make it desolate, even until the consummation that is determined, is poured out on the desolate."
Verse 25 specifically says that when the command is given to rebuild Yeruwshalom there will be 69 weeks before Yahushua the Son of Yahuwah comes on the scene as haMashiach (starts His ministry). Since the entire prophesied time for this is 70 weeks, one week needs to be explained. The followers of the “Great Tribulation” theory believe that week, or even years, is the period of tribulation at the end. If that were true, then there has already been a period of tribulation for 3½ years.
Actually, haMashiach (Yahushua) started His ministry at the end of 69 weeks, just as was prophesied. The prophesy sequence continues to run and Yahushua preached the kingdom for 3½ years before He was killed. When He was impaled 69½ weeks of the prophecy had been fulfilled. Verse 27 gives us the period of His ministry and it is plain that only half of it was accomplished. Verse 26 tells us that He was “cut off” in the middle of the week. This statement is in fact true in two ways. One of the meanings is that His ministry was 3½ years in His first appearance and the other is that the actual day of the week on which He was impaled was the fourth day (middle day of the week). That leaves the other half of His seven year ministry remaining. That this will be a period of tribulation is sure, but it is not “The Great Tribulation” talked about by the adherents of that doctrine. In fact it is the last half of Yahushua’s ministry and it will be fulfilled on the manner of which He stated in MattithYahuw 10:34-36.
Matthew 10:34-36 Do not think that I came to bring peace upon the earth. I did not come to bring peace but a sword. 35 For I have come to set a man against his father, a daughter against her mother, and a daughter-in-law against her mother-in-law, 36 and a man's enemies will be those of his household.
There are many errors of doctrine that are seen in the denominations due to them trying to “exoticize” the scriptures with this theory or that theory. The only way to avoid being pulled in by them is to study scripture for yourself and see if what you are being told is really true. I do not mean just to see if the verses quoted by them are really there, but that the meanings and context they have applied really exist.