Before the Jews left Egypt (in 1513 BC - see [101]) the Hebrew calendar year used to start in the month of Tishri (August/September) see - BLC. After they left Egypt as recounted in the book of Exodus, the Hebrew calendar year was changed by a command from God. (Being God he can do this sort of thing):
1
Jehovah now said to Moses and Aaron in the land of Egypt:2 Now the sons of
Israel should prepare the passover sacrifice at its appointed time.
3 On the 14th day in this month between the 2
evenings you should prepare it at its appointed time [between
sunset and darkness]. According to all its statutes and all its
regular procedures you should prepare it.
4 So Moses spoke to the sons of Israel to
prepare the passover sacrifice.
5 Then they prepared the passover sacrifice
in the first month, on the 14th day of the month between the 2 evenings,
in the wilderness of Sinai. According to all that Jehovah had commanded Moses,
so the sons of Israel did (Numbers 9).
In order sacrifice the lamb on the 14th day and between the two evenings, that day must begin at sunset, not at darkness. Jesus was sacrifice between darkness on Nisan14 and sunset at the end of Nisan14, also between two evenings on Nisan14, but actually between the other two evenings on Nisan14.
38 And this is what you will offer upon
the altar: young rams each a year old, 2 a day constantly.
39 And you will offer the one young ram in
the morning, and you will offer the other young ram between the 2 evenings.
40 And a 10th part of an ephah measure of
fine flour moistened with the 4th of a hin of beaten oil, and a drink offering
of the 4th of a hin of wine, will go for the first young ram.
41 And you will offer the second young ram
between the 2 evenings. With a grain offering like that of the morning and with
a drink offering like its, you will render it as a restful odor, an offering
made by fire to Jehovah [two lambs within one
working day which actually spans two hebrew days].
42 It is a constant burnt offering throughout
your generations at the entrance of the tent of meeting before Jehovah, where I
shall present myself to you people to speak to you there (Exodus 29).
This was the first Passover festival which occurs on Nisan14. The new calendar year started in the month of Nisan (March/April - Spring) for Sacred things (Religious festivals etc) and Tishri (August/September - Autumn) for farming or economic or secular things or fiscal things in what was an agricultural economy. In particular Land Sabbath years, which occurred every 7th year and were a fallow period for the whole nation for that whole year, were still taken as running from Tishri to Tishri. So it is not immediately apparent whether ages and reigns should be calculated in years starting in Nisan or Tishri. Here are the scriptures which provide the answer:
1
And it came about in the four hundred and eightieth year
after the sons of Israel came out from the land of Egypt, in the fourth year, in
the month of Ziv, that is, the second month, after Solomon became king over
Israel, that he proceeded to build the house to Jehovah (1 Kings 6).
Ziv, was Iyyar (April/May), the month after Nisan.
2 Accordingly he started to build in the second month on the
second [day], in the fourth year of his reign (2 Chronicles 3).
This scripture also refers to Solomon's temple. So the second month of the
fourth year of Solomon's reign was Iyyar. So the first month was Nisan, the
preceding month, see - BLC, so reigns are
measured from Nisan. When Hezekiah became king, he cleansed the temple from
Nisan1 to Nisan16, and so was unable to hold the passover on Nisan14, since the
temple was not clean at that time. So he held it on Iyyar14, 30 days later. We
read:
1
Hezekiah himself became king at the age of 25 years, and for 29 years he reigned in Jerusalem. And his mother's
name was Abijah the daughter of Zechariah.
2 And he kept doing what was right in Jehovah's eyes, according to all that David his forefather had
done.
3 He himself, in the first year of his reigning, in the first
month, opened the doors of the house of Jehovah and began to repair them (2
Chronicles 29).
17 Thus they got started on the first [day] of the first month at sanctifying, and on the 8th day of the month they came to the porch of Jehovah; so that they sanctified the house of Jehovah in 8 days, and on the 16th day of the first month they finished (2 Chronicles 29).
However, the king and his princes and all the congregation in Jerusalem resolved to hold the passover in the second month; for they had not been able to hold it at that time, because not enough priests, on the one hand, had sanctified themselves and the people, on the other hand, had not gathered themselves to Jerusalem (2 Chronicles 30:2,3).
So the first month of his reigning, i.e. the first month of his first regnal year, was Nisan. King Ahasuerus of Persia, whilst he was also king of Babylon, measured his reign from Nisan as well:
7 In the first month, that is, the month Nisan, in the twelfth year of King Ahasuerus, someone cast Pur, that is, the Lot (Esther 3).
It is apparent from the above that the first month is always Nisan in the context of reigns of kings of Israel and elsewhere. In fact the decree that Nisan should be the first month (Exodus 12:2) would logically apply to everything unless otherwise stated. And only in the case of Land Sabbaths is there a statement that Tishri should be the first month of the year. So we take ages and reigns as counting from Nisan to Nisan.
The bible counts years in the form: The ninth year of Hoshea the King (2 Kings 18:10). We count them currently as the 1999th year after the birth of Christ (although he was actually born on Tishri 10th 2BC (see [41]). 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. Hence we see:
1 In the third year of the kingship of Jehoiakim the king of Judah, Nebuchadnezzar the king of Babylon came to Jerusalem and proceeded to lay siege to it (Daniel 1).
1 The word that occurred to Jeremiah concerning all the people of Judah in the fourth year of Jehoiakim the son of Josiah, the king of Judah, that is, the first year of Nebuchadrezzar the king of Babylon (Jeremiah 25).
Incidentally Nebuchadnezzar and Nebuchadrezzar were the same chap, known as Nebuchadnezzar II (604 - 564 BC). He appears to be a king in the 3rd year of Jehoiakim, before his first regnal year, which was the 4th year of Jehoiakim. This could have been because the third year of Jehoiakim which was 605 BC (Nisan to Nisan) was his accession year, the year in which he became king, but his predecessor, Nabopolassar, his dad, was still reigning at the beginning of the year and so he got the regnal year rather than his son. Or it could have been because he was inferior coregent with his dad Nabopolassar in the last few years of his father’s reign. This means that although Nebuchadnezzar was king along with his father, the regnal years were all counted only to his father.
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).
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 BC - see [101]), one became one year old on the first Tishri1 of one's life, and remained such until the second Tishri1 of one's life.
A king only gets a regnal year if he is ruling on Nisan1. And if there is a dispute at that time, the winning king gets that year (out of respect). So for example there was no global power between the Romans and the British because none of the intermediate empires showed an undisputed global military superiority. Likewise Caesar Galba, who reigned immediatley after Nero, acceeded but did not last until Nisan1, so he should not be counted as a Caesar.