Periods of 3.5 years are mentioned in various places in the Book of Revelation. In most cases, we can be reasonably confident that they are referring to the second half of Daniel’s 70th Week. However, there is controversy regarding the Two Witnesses.
This period of 42 months in which Jerusalem is trampled by the Gentiles is assuredly the second half of the 70th Week. The Antichrist invades Israel, breaking the covenant he confirmed, half-way through the 70th Week. When he reaches Jerusalem, he commits the Abomination of Desolation, which he is able to do, because by this point he has militarily crushed Israel. His armies then occupy Israel until they are destroyed by Christ during the Day of the LORD.
The Two Witnesses prophesy for 1260 days. Some think this period falls in the first half of the 70th Week, while others believe it occurs during the second half. On the assumption that one of the Witnesses is Elijah, their ministry in any event must precede the Day of the LORD, because Elijah comes before the Day of the LORD (Malachi 4:5).
The Witnesses are slain by the Antichrist after he is empowered by Satan (which happens around the time of Abomination of Desolation – see below). Three days after their murder, they are resurrected during the Day of the LORD, as is indicated by verse 13 following. On that basis, while not dogmatically asserted, the textual evidence suggests that the Two Witnesses begin to prophesy around the time of the Abomination of Desolation (possibly just before), and finish upon or during the Day of the LORD.
In this passage, we see Israel being protected from Satan’s persecution for 1260 days. This is the Great Tribulation, the second half of Daniel’s 70th Week. Up until the invasion of Israel, there is a false peace, fostered by the covenant confirmed by the Antichrist. That peace is shattered when he comes to take Jerusalem and destroy Israel. Remember, Christ warned Israel to flee upon witnessing the Abomination of Desolation. This same period is referenced in 12:14 and is the “short time” of 12:12.
The narrative of Revelation 12 shows us that Satan is cast out of heaven by the Archangel Michael at the mid-point of the 70th Week. Satan’s time is short (3.5 years) because he will be bound and thrown into the Abyss at the Battle of Armageddon, just after the close of the 70th Week (see Revelation 19-20). Comparing Revelation 12 with 2 Thessalonians 2 and Isaiah 14 suggests that it is Satan speaking through the Antichrist at the Abomination of Desolation, following his removal from heaven. The reader is encouraged to consider Reggie Kelley’s thoughts on the “mystery of iniquity”.
Once again we see a period of 42 months, the period in which the Antichrist is permitted to reign, blaspheme, and persecute the saints. This is the second half of the 70th Week, in which the Great Tribulation takes place. The Antichrist is empowered by Satan, after Satan is cast out of heaven (13:2, 4).