I publish this brief explanation as an reply to a question raised by a reader as regards the correct counting of ages and reigns in the Bible. This question was inspired by Emmanuel's posting on the year of the fllod and Noah's 600th year.
Here is my answer:
The bible counts years in the form: The ninth year of Hoshea the King (2 Kings ). We count them currently as the 2008th year after the birth of Christ (although he was actually born on Tishri 10th 2BC.
The biblical count has several problems. Suppose for example that on Nisan1 in a certain year there is no king, because he died in the previous month and everybody is fighting over who will succeed him. Then we have no way of referring to what year we are in!
Or suppose King [a] died on Nisan1 and his successor King [b] was enthroned on Nisan 2. Then that year would be the last regnal year of King [a] for the whole year, because once the year is named, a thing which occurred on Nisan1, it could not be renamed. For king [b], this year is called the accession year, and the next year is called his first regnal year. Actually the cut off point might have been the passover, Nisan14, rather than Nisan1.
Likewise with ages, if you were born in Tishri 39 BC you became one year old in Nisan 38 BC, and the year 38 BC was your first year. This is totally counter intuitive to the modern way of counting, in which as we know, you would be anything from 6 months to 18 months old during 38 BC and you would only be in your first year until Tishri 38BC. The concept of being one year old all the way through your first year is ludicrous in our counting system.
However Noah managed this feat:
11 In the 600th year of Noah’s life, in the second month, on the 17th day of the month, on this day, all the springs of the vast watery deep were broken open and the flood gates of the heavens were opened (Genesis 7).
6 And Noah was six hundred years old when the deluge of waters occurred on the earth (Genesis 7).
Let us carefully examine the Hebrew…
And Noah son of 600 year and the flood he was waters upon the earth (Genesis 7:6 NIVHEOT New International Version Hebrew English Old Testament).
11 In year of 600 year to life of Noah, in the month the second, in 17th day to the month in the day the that, they burst all of springs of great deep and floodgates of the heavens, they were opened (Genesis 7 - NIVHEOT).
So Noah was 600 years old in the 600th year of his life!
Basically the Hebrew baby became one year old on the first Nisan1 of his life, and remained one year old until the second Nisan1 of his life whereupon he became two years old. Although in Noah's day (born in 2971 BC, died in 2021 BC), since years counted from Tishri (August/September) to Tishri, before the Exodus from Egypt (in 1513), one became one year old on the first Tishri1 of one's life, and remained such until the second Tishri1 of one's life.