I think they "borrowed" this calculation from John Brown (1823).
http://www.bibledecoded.com/understanding120.html
and from WTS sources:
Proclaimers book chap. 10 pp. 134-135 Growing in Accurate Knowledge of the Truth/End of the Gentile Times
The matter of Bible chronology had long been of great interest to Bible students. Commentators had set out a variety of views on Jesus? prophecy about "the times of the Gentiles" and the prophet Daniel?s record of Nebuchadnezzar?s dream regarding the tree stump that was banded for "seven times."?Luke 21:24, KJ; Dan. 4:10-17.
As early as 1823, John A. Brown, whose work was published in London, England, calculated the "seven times" of Daniel chapter 4 to be 2,520 years in length. But he did not clearly discern the date with which the prophetic time period began or when it would end. He did, however, connect these "seven times" with the Gentile Times of Luke 21:24. In 1844, E. B. Elliott, a British clergyman, drew attention to 1914 as a possible date for the end of the "seven times" of Daniel, but he also set out an alternate view that pointed to the time of the French Revolution. Robert Seeley, of London, in 1849, handled the matter in a similar manner. At least by 1870, a publication edited by Joseph Seiss and associates and printed in Philadelphia, Pennsylvania, was setting out calculations that pointed to 1914 as a significant date, even though the reasoning it contained was based on chronology that C. T. Russell later rejected.
Then, in the August, September, and October 1875 issues of Herald of the Morning, N. H. Barbour helped to harmonize details that had been pointed out by others. Using chronology compiled by Christopher Bowen, a clergyman in England, and published by E. B. Elliott, Barbour identified the start of the Gentile Times with King Zedekiah?s removal from kingship as foretold at Ezekiel 21:25, 26, and he pointed to 1914 as marking the end of the Gentile Times.
Early in 1876, C. T. Russell received a copy of Herald of the Morning. He promptly wrote to Barbour and then spent time with him in Philadelphia during the summer, discussing, among other things, prophetic time periods. Shortly thereafter, in an article entitled "Gentile Times: When Do They End?", Russell also reasoned on the matter from the Scriptures and stated that the evidence showed that "the seven times will end in A.D. 1914." This article was printed in the October 1876 issue of the Bible Examiner. The book Three Worlds, and the Harvest of This World, produced in 1877 by N. H. Barbour in cooperation with C. T. Russell, pointed to the same conclusion. Thereafter, early issues of the Watch Tower, such as the ones dated December 1879 and July 1880, directed attention to 1914 C.E. as being a highly significant year from the standpoint of Bible prophecy. In 1889 the entire fourth chapter of Volume II of Millennial Dawn (later called Studies in the Scriptures) was devoted to discussion of "The Times of the Gentiles." But what would the end of the Gentile Times mean?