Good question, Room 215, they used the 3 1/2 year (42 months) period in that scripture to explain the captivity concept.
http://wol.jw.org/en/wol/d/r1/lp-e/2014847#h=11:0-19:0
Who are the two witnesses mentioned in Revelation chapter 11?
Revelation 11:3 speaks of two witnesses who would prophesy for 1,260 days. The account then says that the wild beast would “conquer them and kill them.” But after “three and a half days,” these two witnesses would be brought back to life, much to the amazement of all onlookers.—Rev. 11:7, 11.
Who are these two witnesses? The details of the account help us to identify them. First, we are told that they “are symbolized by the two olive trees and the two lampstands.” (Rev. 11:4) This reminds us of the lampstand and two olive trees described in the prophecy of Zechariah. Those olive trees were said to picture “the two anointed ones,” that is, Governor Zerubbabel and High Priest Joshua, “standing alongside the Lord of the whole earth.” (Zech. 4:1-3, 14) Second, the two witnesses are described as performing signs similar to those performed by Moses and Elijah.—Compare Revelation 11:5, 6 with Numbers 16:1-7, 28-35 and 1 Kings 17:1; 18:41-45.
What is the common denominator that links these references? In each case, the account refers to God’s anointed ones who took the lead during a difficult time of testing. So in the fulfillment of Revelation chapter 11, the anointed brothers who took the lead at the time of the establishment of God’s Kingdom in heaven in 1914 preached “in sackcloth” for three and a half years.
At the end of their preaching in sackcloth, these anointed ones were symbolically killed when they were thrown into prison for a comparatively shorter period of time, a symbolic three and a half days. In the eyes of the enemies of God’s people, their work had been killed, causing those opponents much joy.—Rev. 11:8-10.
However, true to the words of the prophecy, at the end of the three and a half days, the two witnesses were brought back to life. Not only were these anointed ones released from prison but those who remained faithful received a special appointment from God through their Lord, Jesus Christ. In 1919 they were among those who were appointed to serve as a “faithful and discreet slave” to care for the spiritual needs of God’s people during the last days.—Matt. 24:45-47; Rev. 11:11, 12.
Interestingly, Revelation 11:1, 2 links these events to a time when the spiritual temple would be measured, or evaluated. Malachi chapter 3 mentions a similar inspection of the spiritual temple, followed by a time of cleansing. (Mal. 3:1-4) How long did this inspection and cleansing work take? It extended from 1914 to the early part of 1919. This period of time includes both the 1,260 days (42 months) and the symbolic three and a half days referred to in Revelation chapter 11.
How happy we are that Jehovah arranged for this spiritual refining work to cleanse a special people for fine works! (Titus 2:14) Additionally, we appreciate the example set by the faithful anointed ones who took the lead during that time of testing and thereby served as the symbolic two witnesses.*
For more information, see The Watchtower of July 15, 2013, page 22, paragraph 12.