Before the Jews left Egypt (in 1513 BC - see U101, 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):
1Jehovah 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 calendar day must begin at sunset, not at darkness. Jesus was sacrificed between darkness on Nisan14 and sunset at the end of Nisan14, also between two evenings on Nisan14. The reason that the Hebrews had two evenings was that they had two days. The 12 hour daylight day ran from first light (before sunrise) to the second evening (darkness). The 24 hour day ran from sunset (the first evening) to sunset. So the first evening (sunset) was the end of their 24 hour day and the second evening (darkness) was the end of their 12 hour day.
The first evening, sunset, is the end of the 24 hour calendar day.
The second evening, darkness, is the end of the 12 hour daylight day.
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, one daylight day, which actually spans two calendar days, two 24 hour 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).
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:
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:
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 Heshvan21 10th 2BC (the 2NC Pentecost, first ripe fruits - see U108 and see U16a) whereas Adam was born on 4027Tishri2, 2NC first fruits - see U41). 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 Nisan2. 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. 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 Nebuchadnezzar's accession year, the year in which he became king, but his predecessor, Nabopolassar, his dad, was still reigning at the beginning of that 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.
Adam was born on 4027Tishri2 - see U41 and see U108, hence the Tishri1 calendar. So ages are counted in Tishri1 years. The age of ones adamic flesh is a secular thing in any event. To the Hebrew one is zero years old until one reaches one's first Rosh Hashana on Tishri1, when one immediately becomes 1 year old. 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!
No human today can mange that. We would all be 9 years old in the 10th year of our life, until we had live 10 years to date, at which point we would become 10 years old..
Not so with the Hebrews. Basically the Hebrew baby became one year old on the first Tishri1 of his life, and remained one year old until the second Tishri1 of his life whereupon he became two years old. Obviously before Moses, 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 U101), 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. Actually it may not be the case that a king gets a regnal year just because he was reigning on Nisan1. Because that is when the new sacred year is given to the king.
But if there is a dispute on Nisan1 as to who is the king. Then people have to be very careful about naming the year, for fear of alienating the winner of the dispute. So in that case the year would normally be given to nobody until the dispute was resolved, The surviving or winning king would then be retroactively awarded the year (out of respect).