I believe that the most accurate Calendar is the Jewish Calendar. The Jews have been in the painstaking habit of recording Genealogies and their accurate records refer back to Adam himself.
Though Adam is directly related to all of the humanity and not to the Jew any more than any other People Group - the Jews were entrusted by God Himself to bear the earliest records of the Ancient People.
Abraham is often referred to as The 'Father of Faith' and rightly so, and though the Jewish Lineage is traced back to Abraham, It was his grandson Jacob who was called; 'Israel'.
Abraham was from the city of Hebron and could most certainly be called a 'Hebrew', or a Hebronite. But it is Jacob, biblically speaking, who is given the name by which the family of the Jews, who would follow, would be called.
Abraham fathered both Ishmael and Isaac. And after the death of Sarah, who died at 127 years of age, when Isaac was 37 years old, Abraham married again at least 3 years after her death.
He married a woman named Keturah. And she bore him six more sons. Zimran. Jokshan. Medan. Midian. Ishbak. Shuah. Though not numbered, Genesis 25:6 informs us that sons were born unto Abraham by the 'Concubines', he had. Note that this is 'plural'.
Genealogies gave us much reliable and valuable information. Not only do they reveal the length of a life, but they often reveal the peculiar 'bloodlines' especially those that would lead directly to the Christ. This I believe was the central purpose of recording genealogies in the first place.
Another benefit, in MY estimation, has been to give us an exacting Calendar of Years since either the 4th Day of Creation or ... the 6th Day.
No other Ancient Nation is so precise in the measuring of Time as that of the Jewish Nation.
It is my 'Personal Belief' that God regards this Calendar, which marks the beginning of Mankind and I believe accentuates the History of Man - even to the Time of his completion on Earth.
We know and believe, according to Scripture, that God will send His Very Son to Rule on the Earth at a future date, for the period of 1,000 Calendar Years. I hold this to be True.
It is also my position that God will regard 6,000 years to be completed BEFORE Christ Jesus returns and Rules for a Millennium. Six Thousand Years of Six Days of Creation. Crowned by One Thousand Years of the Messiah's Reign. For ... the Seventh Day. For Jesus is the Lord of the Sabbath.
The Day of His Return truly draws nigh. But as every generation has considered the possibility of His Return in their day as 'Definite', I hold that a space of time is yet to be fulfilled before that Moment in Prophetic History.
We are in the Year 5777. Consider this well.
Matthew 12:8 For the Son of man is Lord even of the sabbath day. KJV.