Well, lets look into the timeline of the prophecy and what history shows. We find the 1260 year prophesy in the Bible was mentioned seven times, to emphasize it's great importance. But why is it so important, well because it perfectly identifies the Beast or antichrist power and gives understanding the other parts of scripture with prophecy. Here the exact same time in the timeline referred to as 3 1/2 times in Daniel 7 meaning the same period of 1260 days/years exact same period of persecution in church history:
Daniel 7:25 And he shall speak great words against the most High, and shall wear out the saints of the most High, and think to change times and laws: and they shall be given into his hand until a time and times and the dividing of time.
This verse in context is referring to a period of time that the little horn or Antichrist power will persecute the true believers or true church, for 1260 years. The same is true for the time given "that it shall be for a time, times, and an half" for Daniel 12:7
So why is it not a year equal to 365 days as we hold today? Well in ancient times the year was held to be at 360 days. The fact that the prophets understood a year as 360 days is well attested, and can be seen in the prophecies of Daniel and Revelation as seen in the use of "time, times and half a time" (i.e. 1+2+0.5=3.5), "1,260 days" and "42 months". These references represent a period of 1260 years (based on the 360 day Jewish year multiplied by 3.5). Divide 1,260 days by 42 months and you will get a 30-day month, as 12 months of 30 days equals 360-days in a year
These time periods occur eight times in scripture:
Daniel 7:25, "time, times and a half".
Daniel 9:27, "half one set of seven".
Daniel 12:7, "time, times and a half".
Revelation 11:2, "42 months".
Revelation 11:3, "1260 days".
Revelation 12:6, "1260 days".
Revelation 12:14, "time, times and a half".
Revelation 13:5, "42 months".
So we have a measuring stick in history of when this AntiChrist power would be persecuting and basically ruling without restraint in the world. The prophecy in Daniel 7 and 12, Revelation 11, 12 and 13, all mention this same persecuting power with its false system of worship, who enforce a church and state union, causing the kings of the earth to bow down and worship it. Looking at Daniel we understand this can only be the time of the Roman empire and it's change into Papal Rome.
Lets look at the following timeline which outlines just how it began when the Pope first took it's powers over church and state:
533 AD - Emperor Justinian decrees John, as the Bishop of Rome, Chief Bishop of all the churches and he was not to be questioned that in her was the supremacy of the pontificate. Edicts of Novellae, 131st chap. II. This was not the first time that the Bishop of Rome declared himself to be ruler of all the churches, but it is the first time the claim was backed up by the secular sword and temporal powers granted to the Papal System.
536 AD - General Belisarius, under the command of Justinian, marched to Rome, deposed the Ostrogoth installed Bishop of Rome (Silverius) and exiled him.
537 AD - Belisarius installed Vigilius as Bishop of Rome
538 AD - General Belisarius broke the siege and the Ostrogoths withdrew from Rome. Pope Silverius died in prison, marking it the first time Justinians Decree, the supremacy of the Bishop of Rome over the Church could actually be implemented by Vigilius.
1798 AD - General Berthier marched to Rome, entered it unopposed and proclaimed it a Roman Republic, demanded of the Pope the renunciation of his temporal authority, then take prisoner.
The Original Catholic Encyclopedia states on Pope Vigilius beginning pontificate and his temporal powers:
"From this time on the popes, more and more involved in worldly events, no longer belong solely to the Church; they are men of the state, and then rules of the state.[ EDITORS' NOTE : Some writers hold that the beginning of Vigilius' legitimate pontificate should be reckoned from 538 rather than 537, because his deposed predecessor, Silverius, did not die until June, 538." Original Catholic Encyclopedia - Home of the largest Catholic Encyclopedia resource on the web...
The 1260 years of Papal Supremacy is attested to by many sources, not just a few:
Long ages ago, when Rome through the neglect of the Western emperors was left to the mercy of barbarous hordes, Romans turned to one figure for aid and protection and asked him to rule them; and thus in this simple manner, the best title of all to kingly right, commenced the temporal sovereignty of the Popes. And meekly stepping to the throne of Caesar, the Vicar of Christ took up the sceptre to which the emperors and kings of Europe were to bow in reverence through so many ages, from sentiments of respect for the dignity of his office and because he was the only mediator whom they recognized in their almost interminable wars.
Source: Two Romes, by James P. Conry, in The American Catholic Quarterly Review, Volume XXXVI (36), edited by James Andrew Corcoran, Patrick John Ryan, Edmond Francis Prendergast, 1911, pgs. 193-194.
... the right to her Temporal Sovereignty is an integral right of the Church as constituted by her Divine Founder. It has prevailed de facto for more than twelve hundred years, and has been possessed de jure by Divine Natural Law from the beginning of Christianity,... Source: The Victories of Rome and the Temporal Monarchy of the Church, 5th edition, By Kenelm Digby Best, London, 1906, pg. xii.
Rome has been a government under the Popes for some twelve hundred years. Source: Complete Works of the Most Rev. John Hughes, Archbishop of New York, 1866, Volume 2. pg. 778
... In books of all sizes, and from the pulpit of every church, we have been taught from our infancy, that the "beast, the man of sin, and the scarlet whore," mentioned in the Revelations, were names which God himself had given to the Pope; and we have all been taught to believe of the Catholic Church, that her worship was "idolatrous," and that her doctrines were "damnable.".... Now let us put a plain question or two to ourselves, and to these our readers; and we shall quickly be able to form a just estimate of the modesty, sincerity, and consistency of these revilers of the Catholic religion: ―they will not, because they cannot, deny, that this religion was the ONLY CHRISTIAN religion in the world for fifteen hundred years after the death of Christ. They may say, indeed, that for the first three hundred years there was no Pope seated at Rome. But, then, for twelve hundred years there had been; and, during that period, all the nations of Europe, and some part of America, had become Christian, and all acknowledged the Pope as their head in religious matters; and, in short, there was no other Christian Church known in the world, nor had any other ever been thought of. Can we believe, then, that Christ, who died to save sinners, who sent forth his gospel as the means of their salvation, would have suffered a false Christian religion, and no other than a false Christian religion, to be known amongst men all this while? Will these modest assailants of the faith of their and our ancestors assert to our faces, that, for twelve hundred years at least, there were no true Christians in the world? Will they tell us, that Christ, who promised to be with the teachers of his word to the end of the world, wholly left them, and gave up hundreds upon hundreds of millions of people to be led in darkness to their eternal perdition, by one whom his inspired followers had denominated the "man of sin, and the scarlet whore"? Will they, indeed, dare to tell us, that Christ gave up the world wholly to "Antichrist" for twelve hundred years?
Source: History of the Protestant Reformation in England and Ireland, Showing How That Event Has Impoverished and Degraded the Main Body of the People in Those Countries, by William Cobbett, published in 1832 in New York by John Doyle, 12 Liberty-street; and Thomas Doyle, Market-street, Providence R.I., pages 6 and 7.