I'm aware that the Hebrew calendar is lunar based, normally consisting of 12 lunar months and 354.37 days. I'm aware that, to keep the 3 festivals in sync with the seasons, an extra month is added every 2 to 3 calendar years. I am also aware that leap months were more informal back in the ancient days and based on observation, as opposed to being more structured and consistent like how it is now.
My question is: is it possible that, while there were still leap months every now and then, the common understanding of the time measurement of a year back in ancient Israel was simply 12 lunar months? For example, if I were to say "5 years from now" would that be interpreted as 5x12 lunar months or New Moons, or would leap months be considered in that measurement of time?
The reason why I'm asking this is because of Jeremiah's prophecy about the 70 year captivity. Some secular historians would say the captivity lasted from 605 BCE to 537 BCE. That would make 68 solar years, but that's 70 lunar years, or 841 lunar months. Maybe the Israelites at the time added an occasional leap month into their civil calendar, but maybe they also viewed a year as being twelve months. Thus, when Jeremiah prophesied that they would serve Babylon seventy years, they could've interpreted that to mean 70 lunar years. Is this a possibility?