God gave a Revelation to Christ which was written in the Book of Revelation by Apostle John, to inform the Early Church of important Events which were about to happen:

Revelation 1: The Revelation of Jesus Christ, which God gave unto him, to shew unto his servants things which must shortly come to pass...

Among all the things God revealed to Christ, there was the secret Chronology of the 6th and 7th Trumpets.

For the 6th Trumpet, when we add up the numbers given in Revelation 9:15: 1 hour, 24 hours, 30 Days and 12 Months, we get 67. (1+24+30+12 = 67). It means 67 Lunar years or 66.04 Solar Years.

For the 6th Trumpet, not only the Year, but the exact Month and Day are also given in Revelation. You may be familiar with the scene of Revelation 8: the 7 Angels who stand before God were given 7 Trumpets, another Angel came with a "Golden Censer" and offered at the "Golden Altar", the prayers of the Saints with much incense. The Holy of Holies is only open one day each year: the Day of Atonement (10th of Tishri) , this is where the Golden Censer is kept. Later on, the Angel took Fire of the Altar, placed it in the Censer and cast it upon "the Earth". (after the 1st Day of the Feast of Tabernacles, on the 16th of Tishri, offerings by Fire were made to God)

Revelation 8:
3 And another angel came and stood at the altar, having a golden censer; and there was given unto him much incense, that he should offer it with the prayers of all saints upon the golden altar...

Revelation 8:
5 And the angel took the censer, and filled it with FIRE OF THE ALTAR, and cast it into the earth...

In the verses below this is the same scene as described above with the same Angel, but it is shown other Angles of the same Event, the gathering and burning of the Tares, the destruction of the wicked:

Revelation 9:
13 And the sixth angel sounded, and I heard a voice from the four horns of THE GOLDEN ALTAR which is before God,
14 Saying to the sixth angel which had the trumpet, Loose the four angels which are bound in the great river Euphrates.
15 And the four angels were loosed, which were prepared for an hour, and a day, and a month, and a year, for to slay the third part of men.

Revelation 14:18
And another angel CAME OUT FROM THE ALTAR, WHICH HAD POWER OVER FIRE; and cried with a loud cry to him that had the sharp sickle, saying, Thrust in thy sharp sickle, and gather the clusters of the vine of the earth; for her grapes are fully ripe.

That same Event took place on the 16th of Tishri, 2nd Day of the Feast of Tabernacles. After the Day of Atonement when the prayers of the Saints were offered to God with much incense by the Angel, Offerings by fire for the Feast of Tabernacles were made when Fire of the Altar was cast with the Censer upon "the Earth" by the same Angel.

The Children of Israel were told to observe the Feast of Tabernacles after they have gathered their Corn and Wine, (note that the angel in Revelation 14:18 said, her grapes are fully ripe):

Leviticus 23:
36 Seven days ye shall offer an offering made by fire unto the Lord...

Deuteronomy 16:
13 Thou shalt observe the feast of tabernacles seven days, after that thou hast gathered in thy corn and thy wine:

It was revealed to the Early Church that 67 Lunar Years after Christ's Birth, during the Feast of Tabernacles (16th Tishri) the 6th Trumpet would sound. The date was September 21th, 66 AD. On that date Cestius was in Jerusalem with the Roman Army to quell the Rebellion: this was the date of the 6th Trumpet, the Beginning of the end for Jerusalem. (The Romans originally suffered great losses and retreated, after that Nero sent General Vespasian and his son Titus who destroyed Jerusalem in 70 AD)

Flavius Josephus was a prisoner, an eyewitness and a former commandant of the Jewish rebels, he wrote in his Book War of the Jews (very important, note the Date and the corn):

"But now Cestius, observing that the disturbances that were begun among the Jews afforded him a proper opportunity to attack them, took his whole army along with him, and put the Jews to flight, and pursued them to Jerusalem. He then pitched his camp upon the elevation called Scopus, [or watch-tower,] which was distant seven furlongs from the city; yet did not he assault them in three days' time, out of expectation that those within might perhaps yield a little; and in the mean time he sent out a great many of his soldiers into neighboring villages, to seize upon their corn. And on the fourth day, which was the thirtieth of the month Hyperbereteus, [Tisri,] when he had put his army in array, he brought it into the city..."

Related Studies:

The Seventh Trumpet and the End of the World:
https://www.brighteon.com/d5b6016b-cebf-4dcf-90a9-36005814634b

The Seventh Trumpet and Secrets of the Book of Revelation:
https://www.brighteon.com/c5272a02-da6e-44f5-98d8-40cf3bc784bf

Seven Angels and the Seven last Plagues:
https://www.brighteon.com/ebc6f13e-e474-4d23-af81-97f819165a80