I thought I would set it out for lurkers, but more for my own understanding.
Quite simply, there was no zero A.D. The year after 1 B.C., is 1 A.D. Add 2520 years to 607 B.C. and you get 1914 A.D.
There are so many other issues with this calculation, however. www.jwfacts.com does a good job covering them. Issues include:
1. Two types of year in the one calculation
As mentioned above, Russell used a 360 day year to convert "seven times" in Daniel 4:16 to 2520. (ie "seven times" = 7 x 360 = 2520 days) Then when applying Ezekiel 4:6 (day for a year) he in effect relied on a Gregorian claendar year (365.25 day year) to get from 606 BC to 1914 AD.
2. Whoops! no zero AD
That was fixed by simply moving the date of the fall of Jerusalem back one year, to 607 BC.
3. Whoops! Babylon fell in 539 BC
This is SBF's point above. 539 BC doesn't work with Watchtower theology. If Jerusalem was destroyed 70 years beforehand, then 607 BC becomes 609 BC, and 1914 AD becomes 1912 AD. Solution: Fudge it. Make up some stuff to add a couple of years; ie say it would have taken a couple of years for the release to have happened and/or Jews to return. And just ignore that the bible explicitly says the 70 years ended with the fall of Babylon (see Jerimiah 25:11-12) not when a release or return occurs.
4. Whoops! Bible says Jerusalem destroyed 22 years after 70 years commenced
Jerimiah 25:11 says a large group of nations will have to serve the king of Babylon for 70 years. Jerimiah 27:1-6 makes it clear that this period had already commenced by the first year of the reign of Jehoiakim. Jehoiakim reigned for 11 years (2 Kings 23:36). The next king was Jehoiachin, who only reigned 3 months. Zedekiah was the third king and reigned for 11 years (see 2 Kings 24:18), before Jerusalem destroyed.
The problem for Watchtower is that this means the bible is in complete agreement with secular history; ie Jerusalem fell in 587 BC. (The Jews also believe that Jerusalem fell in 587 BC, but what would they know about the Hebrew Scriptures.)
Watchtower's solution: ignore what the bible says; never mention those passages; just make broad statements saying the bible with Watchtower (which is an outrageous lie).