There is no such thing as a ‘decade’ in the ancient calendar. It is a ‘weeks of years,’ in other words, seven. Likewise, there is no such thing as a ‘century.’ It is a ‘Jubilee,’ which is forty-nine years long. Two Jubilees equal ninety-eight years – not one hundred. This is made obvious in the calendar writings of the Dead Sea scrolls.
For example, Genesis 25:7 tells us that Abraham lived 175 years. The book of Jubilees, found among the scrolls, says the same but in a different way. The age of Abraham is described as follows: “He lived three jubilees and four weeks of years, one hundred and seventy five years.” (Jubilees 23:8) Please do the math: (3*49) + (4*7) = 175 years.
This cycle is demonstrated in the prophecy of Daniel 9:24. The ‘Seventy Weeks Prophecy’ as it is so famously known, is equivalent to ten jubilees. Please do the math: 70*7 = 10*49 = 490 years. Notice; if the Jubilee cycle had been fifty, the length of term would have been five-hundred years.
A misunderstanding exists concerning this matter because Leviticus 25:8-11 speaks of a fiftieth year and the Jubilee cycle is thought by some to step in multiples of fifty. Others argue that the fiftieth year must be part of the forty-nine; otherwise the Jubilee cycle would get out of phase with the Sabbath. Divide seven into fifty and see? Try seven into one hundred? It doesn’t work!
The answer is as follows: Sabbaths began 1st Nisan just like every year did, but the fiftieth year was announced with fanfare of trumpets on the seventh month of the forty-ninth year. It started 10th Tishri straddling the last half of the forty-ninth and first half of the first year of the next Sabbatic cycle. The attached diagram demonstrates how it worked.
By correctly understanding the 49-year Jubilee cycle and its ‘overlaid’ 50th year, we discover an ancient calendar formula. The inner workings of this soli-lunar formula is explained in the following linked articles.