Stanley Leathes notes there is a question about measuring the 70-year period:
It is at least curious and interesting to observe that from 606 B.C., the approximate date of the delivery of Jeremiah's prophecy, to 536 B.C., the approximate date of the edict of Cyrus, the first termination of the captivity, was a period of seventy years.
Again, from 598 B.C., the approximate date of Jehoiachin's captivity (2 Kings xxiv. 12), to 528 B.C., the approximate date of the close of the first period of amnesty and of the counter efforts against the decree of Cyrus (Ezra iv. 6), which were virtually a renewal of the captivity, was a period of seventy years. Thus the captivity was eight years in progress, as was also the return. Also from 588 B.C., the approximate date of the completion of the captivity by the destruction of the city and temple (2 Kings xxv. 8), to 518 B.C., the approximate date of the decree of Darius (Ezra v1. 1), which was a second termination of the captivity, was a period of seventy years. Also from 527 B.C., the approximate date of the second renewal of the captivity (Ezra iv. 6), to the decree of Artaxerxes in 457 B.C., which was the close of the captivity, was a period of seventy years.
It is undoubtedly hard to prove or disprove the exactitude of these several periods within a year or so, but for those who insist that the seventy years are a round number it is plainly needless to do so; while, at all events, the reoccurrence of the number seventy in connection with the history of the captivity and the return of the Jews is, to say the least, striking.
We have contemporary evidence from Zech. i. 12 that the Jews of that age were themselves in doubt as to when the seventy years ended; because they did not clearly know from when to date their commencement. It is reserved for us, in looking back over all the history, to see that the very uncertainty attaching to the determination of the seventy years is itself a confirmation of the reality and genuineness of the prophecy concerning them, inasmuch as not in one way only but in several it may be shown to have fulfilled itself...1
He recognizes the issue was knowing when the 70-year period begin. However as Leathes shows, regardless of which date is used, it was fulfilled. This is affirmed by Daniel who recognized both the number of years and a point of fulfillment:
In the first year of Darius the son of Ahasuerus, by descent a Mede, who was made king over the realm of the Chaldeans— in the first year of his reign, I, Daniel, perceived in the books the number of years that, according to the word of the LORD to Jeremiah the prophet, must pass before the end of the desolations of Jerusalem, namely, seventy years. (Daniel 9:1-2) [ESV]
There is good reason to believe the people were able to accurately count the years.
Two methods, one using the natural world, the other using the calendar of the LORD could be used to measure the period of a year.
There is created method for measuring a year:
And God said, “Let there be lights in the expanse of the heavens to separate the day from the night. And let them be for signs and for seasons, and for days and years, and let them be lights in the expanse of the heavens to give light upon the earth.” And it was so. (Genesis 1:14-15)
Celestial objects were created to be signs for years.2 There are four easily identified candidates which mark the period of one year: the summer or winter solstice, or the summer or winter equinox. Therefore, regardless of any human calendar in use in Babylon (or Jerusalem) which might misstate the length of a year, God created a precise method of identifying the completion of a year.
In addition, the Babylonians celebrated the summer solstice with festivals to the god Tammuz so even "non astronomers" in Babylon would know one year had passed.
Feasts of the LORD
The most frequent event on the LORD's calendar is the weekly Sabbath. Since 52 Sabbaths totals 364 days, one year can be measured with reasonable accuracy. As the prophecy is given in years, it would not be necessary to precisely measure the exact number of days, but the weekly Sabbaths could be used to "get closer" to the precise number of days in the entire 70-year period by counting an additional Sabbath every seventh year (corresponding to the Sabbath year in Leviticus 25).
The years could also be counted using any one of the seven annual events:3
However, these annual events are observed using the monthly calendar which seemingly creates potential uncertainty or a "fuzzy timeline" which is at the heart of the OP's question:
...since the lunar year was about 11 days less than the solar year, it was periodically necessary to intercalate a thirteenth month in order that new year's day should not fall before the spring of the year (March-April). No precise details are known of the method used by the Hebrews to accommodate the agricultural with the lunar calendar.4
On the surface there is an arbitrary nature to using the monthly calendar to observe annual events. Yet it is important to remember, there is no celestial object created for the purpose of measuring a month. Nor is there any proscribed way to correlate some number of months to a year. The alignment of the first month of the year to the solar cycle was done by correlating man's work to the created seasons:
“Speak to the people of Israel and say to them, When you come into the land that I give you and reap its harvest, you shall bring the sheaf of the firstfruits of your harvest to the priest, and he shall wave the sheaf before the LORD, so that you may be accepted. On the day after the Sabbath the priest shall wave it. (Leviticus 23:10-11)
The required offering for the Feast of Fruits was the first of the harvest from that year. Thus the monthly calendar was not adjusted at random; it was done to align the first month with the first harvest which was the first season of the year. Using Leathes' terminology, one may debate the exactitude of the specific day on which the year begins and ends, but there is no confusion over the reality a new harvest means a new year.
Moreover, the human activities on Feast of First Fruits effectively function as annual memorials to God's work, the solar cycle, man's annual work and weekly rest:
- It follows the Passover which is marked by offering a year-old lamb and so recognizes the previous year of God's work in providing the lamb and man's care in keeping it.
- It follows the first harvest and so recognizes a new solar cycle.
- It follows the last day of man's work (before the rest of the Sabbath) and recognizes the last day of man's weekly rest.
In other words, the Feast of First Fruits not only marks the period of a year, it does so at a time of year and in a way which connects the new year to the previous year and is grounded both in two aspects of nature (the solar and reproductive cycles) and man's annual work and weekly rest.
There are two valid ways a person can measure a one year period and the only requirement to know the fulfillment is to count seventy 1-year periods. There is a valid question over when one should start to count, but regardless of the starting point, there is a fulfillment 70-years later.
It is impossible to know with certainty which method was used, but the logical candidate would be the Passover and the days of Unleavened Bread which immediately follow which include a weekly Sabbath and the Feast of First Fruits. The Passover is a retrospective look remembering the LORD's work in bringing the Israelites out of bondage in Egypt and would serve prospectively in faith the LORD would end the captivity and bring the Israelites out of Babylon.
Finally, there is another interesting aspect to the connection between the LORD's calendar, the nature solar cycle, and a 70-year period. When 70 years are counted using only the weekly Sabbaths adding a Sabbath every seventh year, the total number of days at the end of 70-years is 25,550, which is 17.5 days short of 70-years calculated using a 365.25 day solar cycle. Thus, in the 70th year, the fulfillment using the solar method would place the end of the captivity immediately after the 17th of the month, which in the first month is one week after the Passover Lamb was chosen and in the seventh month is one week after the Day of Atonement.
Like the different ways to mark the beginning of the 70-years each has a corresponding fulfillment, the natural method of counting has a point of fulfillment marked by a confluence of the weekly 7-day cycle and annual calendar of the LORD.
1. Stanley Leathes, Old Testament Prophecy: Its Witness as a Record of Divine Foreknowledge, Hodder and Stoughton, 1880, pp. 178-179
2. There is no mention of months or objects created to determine the length of a month in the creation account.
3. The New Bible Dictionary, WM. B. Eerdmans Publishing, 1962, p. 177
4. Ibid., p. 178