Jeremiah's seventy years Many people come to different conclusions around the seventy year exile prophesied I. Jeremiah 25:11. Some say they were 360 day prophetic years, others say "seventy years" is a phrase to mean a generation. 

The dates for the captivity vary, ranging from 606 BC to 536 BC. Wikipedia and others say the exile lasted from 605 BC to 537 BC. That's 68 years. However, with those dates I've realised that, although it only makes 68 solar years, it makes 70 lunar years. 

I know the Hebrew calendar is lunar based, with a typical year consisting of twelve lunar months and having an additional 13th month once the first month drifted from Spring (when the first month is supposed to occur). These leap months are now mathematical, consistent, and predictable, but back in the time of ancient Israel these leap months were off the cuff, only occurring of the high priest said so at the end of the twelve months. Therefore I'm assuming there had to be some general idea of what a "year" actually was, time wise. Otherwise people will could make pledges for X years without any knowing of how many months they had pledged for. 

So maybe when Jeremiah prophesied the 70 year exile, the Jews at the time could've understood that as being a time duration of 70 typical years, or 70x12 months.

Is this argument plausible? What are your opinions?

I've looked into the Talmud for answers and it appears that there is a difference between someone making a pledge for "a year" or "this year" and someone making a pledge for "one year". "This/a year" would mean for the rest of the year, even if that year turned into a leap year, whereas "one year" indicates 12 lunar months. I don't know how "seventy years" would be viewed as in the Talmud.
The Hebrew calendar is actually a luni-solar one (https://en.wikipedia.org/wiki/Hebrew_calendar).

It is based not only on the moon, but also by the sun.  The reason is the moon revolves slower around the earth (about 29-30 days) than the earth around the sun (about 365.25 days).  This means that roughly every three years, the Hebrew calendar "added" a month (called intercalary month) of 29-30 days to bring the lunar time back in line with the sun's time.

This lunar/solar necessity is spelled out clearly in Ex. 12:2 (This month shall be unto you the beginning of months: it shall be the first month of the year to you.)  It is the beginning of months (plural).  This, in combination with the barley first fruit wave sheaf offering (Lev. 23:10) and other seasonal feasts.  If they merely followed the moon cycle, within a couple of years there would not be barley to wave, nor to harvest 50 days later at Pentecost (Lev. 23:15-22).  So, they "added" the month to bring "agriculture time" back into the moon and sun alignment so they could follow the commandments.

This necessity and time keeping is also clearly found centuries later with Josephus in his Antiquities of the Jews Book 3 Chapter 10.5  ("5. In the month of Xanthicus, which is by us called Nisan, and is the beginning of our year, on the fourteenth day of the lunar month, when the sun is in Aries,").  The alignment time was thus marked as the spring equinox.  This means they required the new moon and full moon to both be after the Spring Equinox.  The sun would be in Aries and alight the new moon also in Aries.

So, the Seventy Years is in fact 70 years as measured by the rotation past the Spring Equinox and not by the lunar months (30 days or 360 days) alone.