Actually Revelation explains the period of seven years. Here goes...
As you probably know, Revelation is a timeline of a series of events punctuated by parenthetical explanations of the events and characters being portrayed. The book’s time sequence of things is emphasized by the sequential opening of the seals, one after the other, the blowing of the seven trumpets, one after the other, and the pouring out of the vials in order. Probably need to explain all seven years to get the context of the last 3½ years.
1st 3½ years
The beast that ascends out of the bottomless pit is first mentioned in Revelation 11:7 when he kills the two witnesses after their 3½ year testimony.
And I will give power unto my two witnesses, and they shall prophesy a thousand two hundred and threescore days [=3½ years], clothed in sackcloth... And when they shall have finished their testimony, the beast that ascends out of the bottomless pit shall make war against them, and shall overcome them, and kill them. Revelation 11:3,7.
The killing of the two witnesses takes place near the end of the trumpet sound of the sixth angel (the second woe) and just before the sound of the seventh angel (the third woe) in Rev. 11:14. This marks the end of the first 3½ years (what Jesus in Matthew 24:8 called “the beginning of sorrows”). So the first 3½ years are from Rev 6:1 through 11:14.
Second 3½ years
From that point on, after the sounding of the seventh angel (the third woe), this beast is allowed to have power to continue 3½ years (Rev 13:5).
And they worshipped the dragon which gave power unto the beast: and they worshipped the beast, saying, Who is like unto the beast? who is able to make war with him? And there was given unto him a mouth speaking great things and blasphemies; and power was given unto him to continue forty and two months. Rev 13:4-5.
The beast first appears at the end of the first 3½ years but is given "power to continue" during the second 3½ years (Rev 13:5).
The verses just preceding Revelation 11:14 map to “the midst of the week” when he takes away the temple sacrifice (Dan 9:27). God calls this former political world ruler (who “conquers” Rev 6:2) “the beast” beginning with Rev 11:7 because in the second 3½ years, “he as God sits in the temple of God, showing himself that he is God” (2 Thess 2:4) and is therefore worshiped. (Rev 13:4).
The second 3½ years are from Rev 11:15 through 19:21.
God bless
As you probably know, Revelation is a timeline of a series of events punctuated by parenthetical explanations of the events and characters being portrayed. The book’s time sequence of things is emphasized by the sequential opening of the seals, one after the other, the blowing of the seven trumpets, one after the other, and the pouring out of the vials in order. Probably need to explain all seven years to get the context of the last 3½ years.
1st 3½ years
The beast that ascends out of the bottomless pit is first mentioned in Revelation 11:7 when he kills the two witnesses after their 3½ year testimony.
And I will give power unto my two witnesses, and they shall prophesy a thousand two hundred and threescore days [=3½ years], clothed in sackcloth... And when they shall have finished their testimony, the beast that ascends out of the bottomless pit shall make war against them, and shall overcome them, and kill them. Revelation 11:3,7.
The killing of the two witnesses takes place near the end of the trumpet sound of the sixth angel (the second woe) and just before the sound of the seventh angel (the third woe) in Rev. 11:14. This marks the end of the first 3½ years (what Jesus in Matthew 24:8 called “the beginning of sorrows”). So the first 3½ years are from Rev 6:1 through 11:14.
Second 3½ years
From that point on, after the sounding of the seventh angel (the third woe), this beast is allowed to have power to continue 3½ years (Rev 13:5).
And they worshipped the dragon which gave power unto the beast: and they worshipped the beast, saying, Who is like unto the beast? who is able to make war with him? And there was given unto him a mouth speaking great things and blasphemies; and power was given unto him to continue forty and two months. Rev 13:4-5.
The beast first appears at the end of the first 3½ years but is given "power to continue" for the next 3½ years (“42 months”) (Rev 13:5).
The verses just preceding Revelation 11:14 map to “the midst of the week” when he takes away the temple sacrifice (Dan 9:27). This former political world ruler (who “conquers” Rev 6:2) is what scripture calls “the beast” beginning with Rev 11:7 because in the second 3½ years, “he as God sits in the temple of God, showing himself that he is God” (2 Thess 2:4) and is therefore worshiped. (Rev 13:4).
The second 3½ years are from Rev 11:15 through 19:21.