It would seem that none of the respondents here know WHERE the 7 years tribulation is scripturally defined.
It centers around the book of Daniel.
Daniel 9:24-27
24 Seventy weeks are determined upon thy people and upon thy holy city, to finish the transgression, and to make an end of sins, and to make reconciliation for iniquity, and to bring in everlasting righteousness, and to seal up the vision and prophecy, and to anoint the most Holy.
25 Know therefore and understand, that from the going forth of the commandment to restore and to build Jerusalem unto the Messiah the Prince shall be seven weeks, and threescore and two weeks: the street shall be built again, and the wall, even in troublous times.
26 And after threescore and two weeks shall Messiah be cut off, but not for himself: and the people of the prince that shall come shall destroy the city and the sanctuary; and the end thereof shall be with a flood, and unto the end of the war desolations are determined.
27 And he shall confirm the covenant with many for one week: and in the midst of the week he shall cause the sacrifice and the oblation to cease, and for the overspreading of abominations he shall make it desolate, even until the consummation, and that determined shall be poured upon the desolate.
Throughout prophetic scripture, the term 'weeks' actually refers to 7 years = 1 week. Therefore, 70 weeks = 490 years.
Verse 25 shows the time from the King Cyrus' commandment to restore Jerusalem (after Israels' 70 year Babylonian captivity) consists of 69 weeks (7+62 = 483 years) between the command given to the cross (verse 26). So everyone is waiting for the 70th week to happen, eg, the tribulation.
In verse 27, the 70th week begins...Daniel doesn't identify who 'he' is, but it's clearly the anti-christ. Reference to the midst of the week (3.5 years). After the first 3.5 years, the anti-christ will defile the temple, which many think that he will slaughter a pig on the altar. But note that there has to be a temple built first! It's fairly common knowledge that Israel has pretty much a 'pre fab' temple ready to be assembled in its rightful place on the temple mount on short notice. Various temple fixtures, robes, etc, are also ready to go, from what I've read. Then shall the abominations from the anti-christ begin. This prophecy was spoken of by Jesus in Matthew 24 as well as Mark 13.
It's commonly thought that the destruction of the temple in Matthew 24:2 happened in 70 AD. Various denominations further believe that all the rest of Matthew 24 & 25 happened at that time as well. Nothing could be further from the truth. Simply look at what is being described there. Have wars, famines, etc happened yet? Many would say yes. But look at verse 6:
Matthew 24:6 And ye shall hear of wars and rumours of wars: see that ye be not troubled:
for all these things must come to pass, but the end is not yet. (underline mine)
Has the 'end' happened? And, of course, Jesus confirms Daniels' prophecy -
Matthew 24:15 When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand
Various Christian denominations teach that they will have to endure through the 7 years tribulation. The problem is that they fail to realize just how bad it will be. Zechariah 13:8 shows that 2/3 of the world population will die during that time. Yes, Zechariah was speaking/writing to Israel only - as were all the prophets, including Jesus - but as Israel goes, so the world goes. Given todays' technology, it's easy to understand that nuclear weapons will be used for much of the destruction.
Many feel that they will be judged like the sheep and goats in Matthew 25 at the end of the 7 years. Does anyone believe for a moment that they could survive all 7 years of tribulation? Most likely, there will be no electricity as most generation stations can be destroyed via the internet (it's been done on a very small basis worldwide, even a small power dam in the northeast US). What about anyone who requires medications to survive? Good luck getting insulin and keeping it cold, for example.
Of course, the solution for us is salvation by Gods' grace through faith in the death, burial and resurrection of the Lord Jesus Christ per the Gospel as given to and relayed by Paul:
1 Corinthians 15:1-4
1 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
4 And that he was buried, and that he rose again the third day according to the scriptures:
Faith plus nothing.
We will all be 'snatched away' in the rapture of the believing church (not denominations, but all true believers) BEFORE the tribulation begins!
1Thessalonians 4:13-18
13 But I would not have you to be ignorant, brethren, concerning them which are asleep, that ye sorrow not, even as others which have no hope.
14 For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him.
15 For this we say unto you by the word of the Lord, that we which are alive and remain unto the coming of the Lord shall not prevent them which are asleep.
16 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
17 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
18 Wherefore comfort one another with these words.