Why did God, through Jeremiah, decree 70 years of Babylonian captivity for His people, as Daniel learned in Daniel 9:2?
According to Leviticus 26:34, 35 and 2 Chronicles 36:21, the length of the captivity was to allow the land to enjoy all the sabbath-rest years it had not been allowed during Israel’s years of apostasy.
How many years of apostasy would it take to miss 70 sabbath-rest years? Every seventh year was to be a rest year for the land (Leviticus 25:4). In addition, on every 50th year (Leviticus 25:10, 11), called the jubilee, the land was to rest as well. That 50th year was actually the first year of the next 49-year cycle.
With 8 sabbath-rest years (7 sabbatical years plus 1 jubilee year) to be observed for every jubilee cycle of 49 years, it would mathematically take at least 8.75 jubilee cycles, or 428.75 years, to accumulate 70 sabbath-rest years. The period of apostasy would have to be fewer than 434.875 years (which is 8.875 jubilee cycles), because that would produce 71 sabbath-rest years. So for them to have missed exactly 70 Sabbath-rest years, their period of apostasy had to last somewhere in the range from 429 to 434 years.
Do we have a record of how many years of apostasy God’s people experienced? We do. In Ezekiel 4:4-6, the prophet was told to lie on his left side and bear the iniquity of the house of Israel 390 days. Then he was to lie on his right side and bear the iniquity of the house of Judah 40 days. “For I have laid upon thee the years of their iniquity” (v. 5). “I have appointed thee each day for a year” (v. 6). Thus the total period of the iniquity of Israel and Judah combined was 430 years. That would have resulted in exactly 70 sabbath-rest years that they ignored. Those untaken years of rest were now to be given to the land. God’s people would be exiled to Babylon for seventy years (Jeremiah 25:11, 12; 29:10).
This can be calculated in more than one way. For example, the ratio of Sabbath-rest years to regular years (8 to 49) is 1 to 6.125. Apply that ratio to the 430 years and round it to the nearest whole year: 430 ÷ 6.125 = 70.2 = 70 years.
Another way is to see the 8 sabbath-rest years per 49-year jubilee cycle as 16.3265306% of the time. The formula would then look like this: 430 x 0.163265306 = 70.2 = 70 years.