This is not true, it was not EGW who made the prediction, she was not an Adventist them, she was a Methodist and 17 years old. It was William Miller who made the prediction who was a Baptist minister and he got the timeline right, just the prophecy wrong, the timeline came from God through Scripture and He promises some will understand it, it was about the cleansing of the sanctuary in heaven, not the Second Coming of Jesus Christ, which you are correct no man will know the day. why the book referred to in Revelation about Daniel was sweet in the mouth but became bittersweet. However Jesus gave us prophecy so we would know the signs of His soon return.
This. Read The Great Controversy, Chapter 18 - An American Reformer. Ellen explains how William Miller determined what he did concerning the 2300 day prophecy. Here's an excerpt.
The prophecy which seemed most clearly to reveal the time of the second advent was that of Daniel 8:14: “Unto two thousand and three hundred days; then shall the sanctuary be cleansed.” Following his rule of making Scripture its own interpreter, Miller learned that a day in symbolic prophecy represents a year ( Numbers 14:34; Ezekiel 4:6); he saw that the period of 2300 prophetic days, or literal years, would extend far beyond the close of the Jewish dispensation, hence it could not refer to the sanctuary of that dispensation. Miller accepted the generally received view that in the Christian age the earth is the sanctuary, and he therefore understood that the cleansing of the sanctuary foretold in Daniel 8:14 represented the purification of the earth by fire at the second coming of Christ. If, then, the correct starting point could be found for the 2300 days, he concluded that the time of the second advent could be readily ascertained. Thus would be revealed the time of that great consummation, the time when the present state, with “all its pride and power, pomp and vanity, wickedness and oppression, would come to an end;” when the curse would be “removed from off the earth, death be destroyed, reward be given to the servants of God, the prophets and saints, and them who fear His name, and those be destroyed that destroy the earth.”—Bliss, page 76. { GC 324.3}
With a new and deeper earnestness, Miller continued the examination of the prophecies, whole nights as well as days being devoted to the study of what now appeared of such stupendous importance and all-absorbing interest. In the eighth chapter of Daniel he could find no clue to the starting point of the 2300 days; the angel Gabriel, though commanded to make Daniel understand the vision, gave him only a partial explanation. As the terrible persecution to befall the church was unfolded to the prophet’s vision, physical strength gave way. He could endure no more, and the angel left him for a time. Daniel “fainted, and was sick certain days.” “And I was astonished at the vision,” he says, “but none understood it.” { GC 325.1}
Yet God had bidden His messenger: “Make this man to understand the vision.” That commission must be fulfilled. In obedience to it, the angel, some time afterward, returned to Daniel, saying: “I am now come forth to give thee skill and understanding;” “therefore understand the matter, and consider the vision.” Daniel 8:27, 16; 9:22, 23, 25-27. There was one important point in the vision of chapter 8 which had been left unexplained, namely, that relating to time—the period of the 2300 days; therefore the angel, in resuming his explanation, dwells chiefly upon the subject of time: { GC 325.2}
“Seventy weeks are determined upon thy people and upon thy Holy City.... 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. And after threescore and two weeks shall Messiah be cut off, but not for Himself.... 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.” { GC 326.1}
The angel had been sent to Daniel for the express purpose of explaining to him the point which he had failed to understand in the vision of the eighth chapter, the statement relative to time—“unto two thousand and three hundred days; then shall the sanctuary be cleansed.” After bidding Daniel “understand the matter, and consider the vision,” the very first words of the angel are: “Seventy weeks are determined upon thy people and upon thy Holy City.” The word here translated “determined” literally signifies “cut off.” Seventy weeks, representing 490 years, are declared by the angel to be cut off, as specially pertaining to the Jews. But from what were they cut off? As the 2300 days was the only period of time mentioned in chapter 8, it must be the period from which the seventy weeks were cut off; the seventy weeks must therefore be a part of the 2300 days, and the two periods must begin together. The seventy weeks were declared by the angel to date from the going forth of the commandment to restore and build Jerusalem. If the date of this commandment could be found, then the starting point for the great period of the 2300 days would be ascertained. { GC 326.2}
In the seventh chapter of Ezra the decree is found. Verses 12-26. In its completest form it was issued by Artaxerxes, king of Persia, 457 B.C. But in Ezra 6:14 the house of the Lord at Jerusalem is said to have been built “according to the commandment [“decree,” margin] of Cyrus, and Darius, and Artaxerxes king of Persia.” These three kings, in originating, reaffirming, and completing the decree, brought it to the perfection required by the prophecy to mark the beginning of the 2300 years. Taking 457 B.C., the time when the decree was completed, as the date of the commandment, every specification of the prophecy concerning the seventy weeks was seen to have been fulfilled. { GC 326.3}
“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”—namely, sixty-nine weeks, or 483 years. The decree of Artaxerxes went into effect in the autumn of 457 B.C. From this date, 483 years extend to the autumn of A.D. 27. (See Appendix.) At that time this prophecy was fulfilled. The word “Messiah” signifies “the Anointed One.” In the autumn of A.D. 27 Christ was baptized by John and received the anointing of the Spirit. The apostle Peter testifies that “God anointed Jesus of Nazareth with the Holy Ghost and with power.” Acts 10:38. And the Saviour Himself declared: “The Spirit of the Lord is upon Me, because He hath anointed Me to preach the gospel to the poor.” Luke 4:18. After His baptism He went into Galilee, “preaching the gospel of the kingdom of God, and saying, The time is fulfilled.” Mark 1:14, 15. { GC 327.1}
“And He shall confirm the covenant with many for one week.” The “week” here brought to view is the last one of the seventy; it is the last seven years of the period allotted especially to the Jews. During this time, extending from A.D. 27 to A.D. 34, Christ, at first in person and afterward by His disciples, extended the gospel invitation especially to the Jews. As the apostles went forth with the good tidings of the kingdom, the Saviour’s direction was: “Go not into the way of the Gentiles, and into any city of the Samaritans enter ye not: but go rather to the lost sheep of the house of Israel.” Matthew 10:5, 6. { GC 327.2}
“In the midst of the week He shall cause the sacrifice and the oblation to cease.” In A.D. 31, three and a half years after His baptism, our Lord was crucified. With the great sacrifice offered upon Calvary, ended that system of offerings which for four thousand years had pointed forward to the Lamb of God. Type had met antitype, and all the sacrifices and oblations of the ceremonial system were there to cease. { GC 327.3}
The seventy weeks, or 490 years, especially allotted to the Jews, ended, as we have seen, in A.D. 34. At that time, through the action of the Jewish Sanhedrin, the nation sealed its rejection of the gospel by the martyrdom of Stephen and the persecution of the followers of Christ. Then the message of salvation, no longer restricted to the chosen people, was given to the world. The disciples, forced by persecution to flee from Jerusalem, “went everywhere preaching the word.” “Philip went down to the city of Samaria, and preached Christ unto them.” Peter, divinely guided, opened the gospel to the centurion of Caesarea, the God-fearing Cornelius; and the ardent Paul, won to the faith of Christ, was commissioned to carry the glad tidings “far hence unto the Gentiles.” Acts 8:4, 5; 22:21. { GC 328.1}
Thus far every specification of the prophecies is strikingly fulfilled, and the beginning of the seventy weeks is fixed beyond question at 457 B.C., and their expiration in A.D. 34. From this data there is no difficulty in finding the termination of the 2300 days. The seventy weeks—490 days—having been cut off from the 2300, there were 1810 days remaining. After the end of 490 days, the 1810 days were still to be fulfilled. From A.D. 34, 1810 years extend to 1844. Consequently the 2300 days of Daniel 8:14 terminate in 1844. At the expiration of this great prophetic period, upon the testimony of the angel of God, “the sanctuary shall be cleansed.” Thus the time of the cleansing of the sanctuary—which was almost universally believed to take place at the second advent—was definitely pointed out. { GC 328.2}
It wasn't just William Miller who studied the time prophecies of Daniel. Adventist pioneers J. N. Andrews, Uriah Smith, and even James White wrote in great detail on the subject. Here are links to those PDF's.
The Sanctuary and Twenty-three Hundred Days, J. N. Andrews
The 2300 Days and the Sanctuary, Uriah Smith
The Sanctuary, the 2300 Days, and the Shut Door, James White
The Sanctuary and the Twenty-three Hundred Days of Daniel 8_14, Uriah Smith
