Hi Doug.
As St. George said, the 2520 years commences when Zedekiah was removed. That period is to do with the absence of a Davidic king until Jesus' enthronement in 1914.
On the other hand, the 70 years is to do with how long the land was 'desolate, without an inhabitant' which, as the WTS maintains, started from Tishri 607 BCE (the flight to Egypt) to Tishri 537 BCE (the resettlement in Israel).
Further confusion reigns, since according to Parker and Dubberstein, in 607 BCE the Babylonians inserted an intercalary second Elul before Tishri. From memory, Thiele says that Jeremiah used the Babylonian calendar.
Ahh but the intercalary months are attached to kings' regnal years, not BCE dates. No intercalary month is attested to Nebuchadnezzar's 18th year.
As an aside, the intercalary Ululu/Elul (6th month) assigned to 607/6 BCE (really Nabopolassar's 19th year) is an error. See note 9 on p. 4 of P&D as to the uncertainty about this cited tablet. There are actually 5 tablets attesting to Nabopolassar's 18th year having an intercalary Ululu/Elul (C. B. Walker, CBT).