Hi,
The history and the Bible made it quite clear on this and the spirit of prophecy further clarify on it.
In 33AD, when the disciples asked Jesus about the signs of His coming, He answered with the dual fulfillment of the prophecy. This is recorded in Matt 24, Mark 13 and Luke 21.
Jesus said,
Luke 21
20And when ye shall see Jerusalem compassed with armies, then know that the desolation thereof is nigh.
21Then let them which are in Judaea flee to the mountains;...
So from above we know the abomination of desolation and sign to flee have something to do with the compassing enemy army.
If you study that part of history, from the First Jewish-Roman war, the Jews rebelled and expelled the Romans from Jerusalem in 66AD. Rome sent its army to suppress the rebellion. Jerusalem was under siege. But in 68AD, Nero died. The Roman army retreated. The Roman army soon returned with a new general Titus and the city was under siege the second time. In 70AD, Jerusalem was finally sacked and the temple was destroyed.
But according to the account in Great Controversy, not one Christian perished in 70AD because they had fled the city after the first siege when the Roman army retreated. So you see the sign to flee the city can not be the destruction of the temple. It has to be before that.
The spirit of prophecy confirms the timing of their flight:
"Not one Christian perished in the destruction of Jerusalem. Christ had given His disciples warning, and all who believed His words watched for the promised sign. "When ye shall see Jerusalem compassed with armies," said Jesus, "then know that the desolation thereof is nigh. Then let them which are in Judea flee to the mountains; and let them which are in the midst of it depart out." Luke 21:20, 21. After the Romans under Cestius had surrounded the city, they unexpectedly abandoned the siege when everything seemed favorable for an immediate attack. The besieged, despairing of successful resistance, were on the point of surrender, when the Roman general withdrew his forces without the least apparent reason. But God's merciful providence was directing events for the good of His own people. The promised sign had been given to the waiting Christians, and now an opportunity was offered for all who would, to obey the Saviour's warning. Events were so overruled that neither Jews nor Romans should hinder the flight of the Christians. Upon the retreat of Cestius, the Jews, sallying from Jerusalem, pursued after his retiring army; and while both forces were thus fully engaged, the Christians had an opportunity to leave the city. At this time the country also had been cleared of enemies who might have endeavored to intercept them. At the time of the siege, the Jews were assembled at Jerusalem to keep the Feast of Tabernacles, and thus the Christians throughout the land were able to make their escape unmolested. Without delay they fled to a place of safety--the city of Pella, in the land of Perea, beyond Jordan." --- Great Controversy, p31, 32
And the spirit of prophecy further clarifies what that sign, the abomination of desolation is:
"And the Saviour warned His followers: "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

then let them which be in Judea flee into the mountains." Matthew 24:15, 16; Luke 21:20, 21. When the idolatrous standards of the Romans should be set up in the holy ground, which extended some furlongs outside the city walls, then the followers of Christ were to find safety in flight." --- Great Controversy, p26
So the abomination of desolation which is a sign to flee the city was the idolatrous standards (flags) set up in the holy ground, outside the city walls, not the actual destruction of the temple.
Adventists understand the latter fulfillment of this prophecy (sign to flee the cities into the country side) to be the enactment of the National Sunday law.
The Hebrew word
oth means the standard or the flag of an army. It is also as the
seal of God or the
Mark of beast. We know the 7th-day sabbath is a flag, a standard that differentiate the two groups who both claim to worship the same God, Ezekiel 20, Exodus 31.
The spirit of prophecy tells us the assumption of power in decree to enforce the counterfeit sabbath (Sunday) would be the abomination of desolation that is a sign for the true sabbath-keeping Christians to flee the cities:
"
The time is not far distant, when, like the early disciples, we shall be forced to seek a refuge in desolate and solitary places.
As the siege of Jerusalem by the Roman armies was the signal for flight to the Judean Christians, so
the assumption of power on the part of our nation, in the decree enforcing the papal sabbath, will be a warning to us. It will then be time to leave the large cities, preparatory to leaving the smaller ones for retired homes in secluded places among the mountains." --- Country Living, p32, 1885.
Most Adventists you ask today will tell you the time to leave the cities will be in the future when the National Sunday law is passed. But in fact that sign had already came.
Ellen White in 1897 wrote:
"
The Protestant world have set up an idol sabbath in the place where God's Sabbath should be, and they are treading in the footsteps of the Papacy. For this reason I see the necessity of the people of God moving out of the cities into retired country [places,] where they may cultivate the land and raise their own produce. Letter 90, 1897. {CL 21.1}
She continued to write in 1903 that the time has come.
“
The time has come, when, as God opens the way, families should move out of the cities.” Manuscript 50, (1903)
What happened was that in 1888, senator Blair introduced a bill to enforce Sunday keeping on federal territories. Though the bill was defeated, that was the first siege and the sign to flee the city. As we remember from history when 2nd siege came, the Roman army returned with vengeance. More than one million Jews died from starvation and from the conquest. The city was burnt down and the temple was destroyed. Thousands upon thousands more were crucified later.
Hope this helps. You can read more on this here:
http://cherrypicker.tripod.com/OutOfTheCities.pdf
God bless!