When one gathers all available bits of information from various sources including the chronological dating within the bible itself.
It becomes evident that 605 BCE. was the year that Jerusalem was annexed by Babylon via Nebuchadnezzar II, the same year he
became ruler/King upon his father's death. The start of the exile to Babylon also started within that year.
Likewise the finally destruction of Jerusalem and the Temple by Nebuchadnezzar II did occur in 586 BCE.
It should also be noted that there is archaeological proof by way tablets and cylinders biographically dating events pertaining to
Nebucbhnezzar II in support toward other sources, including the bible.
You can play games with pushing a year forward or backward all you want but you cant fight against practical evidence.
What year was the year that exiles returned to Jerusalem isn't exactly clear. 538, 537, 536 ???
We do know of what year the Decree of Cyrus was handed down the same year he over thorough Babylon (538).
Irregardless 607 or 609 shouldn't be an important factor weighing upon the evidence of 605.
One might surmise that since the captivity from the start to the eventual release from Babylon accumulated close to 70 years in approximation,
that a post- prophecy was told to support the power and controlling direction of their god of the ancient Israelites (YHWH)
7 being the most sacred of numbers concerning this god and venerated by this ancient civilization.
It is prudent to be intellectually honest when evaluating the historical evidence of humanity's history, isn't that right Ethos ?
By the way it should be mentioned that if you subtract 70 years from 605 you end up at 535 BCE, 3 years after the proposed Decree
of Cyrus. Does it seem plausible that people would wait 3 years after they were freed to return to their homeland ? ? ?
Or they would subjectively wait 3 years to substantiate a prophecy that was set 100 years prior to their captivity ?
These are real questions where one has to apply logic with a bit of intellectual honesty to derive at a probable conclusion.