The Tewahdo Orthodox church bases its calculation of Earth's age from the 'Book of Jubilees' also known as the 'Book of Divisons' which is part of the Tewahdo Orthodox Church's cannon.
The book begins with God commanding Moses to ascend mount Sinai and receive the two tablets containing the Ten Commandments. For example, in Jubilees 1:28, It is written:"And the angel of the presence who went before the camp of Israel took the tables of the divisions of the years -from the time of the creation- of the law and of the testimony of the weeks of the jubilees, according to the individual years, according to all the number of the jubilees [according, to the individual years], from the day of the [new] creation when the heavens and the earth shall be renewed and all their creation according to the powers of the heaven, and according to all the creation of the earth, until the sanctuary of the Lord shall be made in Jerusalem on Mount Zion, and all the luminaries be renewed for healing and for peace and for blessing for all the elect of Israel, and that thus it may be from that day and unto all the days of the earth
Moreover, according to the book one day in heaven corresponds to 1000 years on Earth.In the book , God promised man, "When five and a half days are full, I will be born of your offspring and save you".
.Thus, 5.5 days make 5500 years until the birth of our Lord Jesus Christ.
Below is an interesting article that might be relevant to the topic of discussion. I apologise for its length.
*****************************************************************************"But when the time had fully come, God sent His only begotten Son..." Gal.4: 4
How do the Ethiopian and the European (Gregorian) calendar differ?
The difference is both in years and days. According to the Ethiopian calendar, the current year is 2001, which is clearly Seven (eight) years behind the Gregorian. The Ethiopian months are also lagging by seven, eight, nine or ten days depending on where the two calendars' months match. We have 12 months with 30 days each and a 13th month with five or six days. The 13th month is called "Pagumen" to mean the thirteenth. It is either six or five days whether the year is leap year. The hours of the day are not named and divided in the same way as in the European. For example, the European say 12 O'clock (AM) when it is actually 6 at midday. In the Ethiopian evenings are considered parts of the next day. In this article, it is attempted to give scriptural background to the 7/8 years difference in the calendars according to the teachings of the Ethiopian Orthodox Tewahedo Church.
The difference lies in determining the exact date of the birth of Christ. According to Dionasius, a Roman monk, the birth of our Lord Jesus Christ was 753 years after the foundation of the city of Rome. It is according to his calculation that the world joins the second millennium, in spite of the fact that, researchers many years after him have already discovered he has made a mistake by at least four years. Their argument was based on the Biblical clues given at Mt. 2:1 and Lk.3: 1-3, 22-23. In Mt. 2.1, it was mentioned that Jesus was born during the time of King Herod. The King died just after the birth of Christ. Meanwhile, it was also recorded that the king died 750 years after the city of Rome was founded. The other Gospel tells us that Jesus was 30 by the time he began his ministry. It was also mentioned that this was the fifteenth year of the reign of Tiberius Caesar. Historians proved that Tiberius came to power 765 years after Rome was founded. Fifteenth of his reign would be in 780, clearly showing a difference of four years from the calculation Dionasius has done. Thus the current European year would have been utmost 2005 AD only!
One would ask, "Why didn't they make these corrections? Why so far? ". One reason for this could be, that based on this calendar many historical records and other important dates and festivals have already been established. Correction would have meant losing of all these documents and reestablishing the dates, which might have created much conflict.
The Ethiopian Christians didn't base themselves on historical facts to decide the date on which Jesus was born. They used ages mentioned in many parts of the Holy Bible. According to their argument, God has nothing to do without reason. He must have meant something when He makes mention of these ages. If one contemplates on this he would draw a remarkable conclusion about this matter.
Moreover, the birth of our Lord was foretold. The time was also fixed. Jesus was not born suddenly. When was it? It is really a big question. How did the Magi come to Jerusalem to worship the infant Jesus? Indeed, they were wise men, and had the prophecy in their tradition that the King of the Jews, whose Kingdom will last forever, will be born in the holy land. It is also important to observe that the chief priests of Jerusalem were able to tell where he was born (Mt.2: 1-5). They knew the date (Dan. 9:24) and place (Mic.5:2). So we can be sure that there is sufficient information to fix the exact date just by following the stories in the Holy Bible and the prophecies.
From the St. Paul's, "When the time had fully come, God sent His only begotten son, born of a woman..." (Gal. 4:4) it was clearly indicated that there was a time period which human beings were supposed to wait. This was, according to Ethiopians, 5500 years. The same was mentioned in the tale of Abraham. This was a covenant given to Adam by God. When Adam was banished from Garden of Eden, he sought to get mercy of God. God told Adam that forgiveness of sin would not be obtained by his effort. He then promised him, "When five and a half days are full, I will be born of your offspring and save you". Note that one-day is one thousand years with God (2Pt.3: . When this time (5500 years) had fully come, Jesus was born.
Then the Ethiopian started to calculate this time following the pace of Biblical history from Genesis up to the time of Jesus the Christ.
The task was not so easy. It requires a good understanding of how the times were mentioned in the Holy Book. It is presented below in a simplified form. The story begins in the fifth chapter of Genesis: (Gen. 5:3)
* When Adam became the father of Seth he was 230
* When Seth became the father of Enosh he was 205
* When Enosh became the father of Kenan he was 190
* When Kenan became the father of Mahalalel he was 170
* When Mahalalel became a father of Jared he was 165
* When Jared became the father of Enoch he was 162
* When Enoch became the father of Methuselah he was 165
* When Methuselah became the father of Lamech he was 187
* When Lamech became the father of Noah he was 182
* The flood came at the 600 age of Noah 600
Sum 2256 years
Now, we finished the fifth chapter of Genesis. Let us now go to the 11th chapter of Genesis. Remember that here Shem's was selected only because Jesus Christ was in his line of generation.(Gen. 11:10)
* When Shem became the father of Arphaxad 2 years after the flood 002
* When Arphaxad became the father of Cainan he was 135
* When Cainan became the father of Shelah he was 130
* When shelah became the father of Eber he was 130
* When Eber became the father of Peleg he was 134
* When Peleg became the father of Reu he was 130
* When Reu became the father of Serug he was 132
* When Serug became the father of Nahur he was 130
* When Nahor became the father of Terah he was 109
* When Terah became the father of Abram, Nahor and Haran, he was 100
* At 75 Abraham went to exile from his relatives and family 075
The grand sum will be 2256 + 1207 = 3463 years. These above numbers are clearly presented and need no deep knowledge of the whole story of the Old Testament. The remaining ones are found scattered so require a rather better knowledge.
From the 75th age of Abraham to Exodus 430 years
From Exodus to Samuel 513 years
From Saul to Solomon 120 years
From Solomon to exile to Babylon 394 years
And in exile 70 years
Sum 1527 years
These sums up to 1527 and 3463 = 4990 years From here on the calculations are based on the verse on the Book of Daniel.(Dan. 9:24-25) In this verse the coming of the Messiah has been clearly stated as 62 sevens'. It goes, "From the issuing of the decree to restore and rebuild Jerusalem until the Anointed One, the ruler, comes, there will be seven 'sevens', and sixty-two 'sevens' ". The first seven 'sevens' which will amount will be 49 years and the sixty-two others will be 434 years. It is also mentioned that Ezra went to Jerusalem with the exile captives of Israel to build Jerusalem and the Temple at the seventh year of the reign of Artaxees. (Ezra7: 6-9). The first sevens will be from the issuing of the word to the end of the rebuilding of the temple. Mind you, the temple has taken 46 years to rebuild (John (2:20) and the three year difference is the time when the Israeli were banned from building (Ezra 1:1-6 & 23-24).
There is one subtle point to understand here. The sixty-two and the seven 'sevens' are not continuous. The beginning of this is the seventh year of the reign of Artaxees king of Persia (Ezra7: 6-9). This can be historically proved that it was the 76th year of the kingdom of Persia which includes the first seven 'sevens'. Therefore, these 76 years plus 434 (=62 x 7) it would be 510 years.
Finally, we reached the point. This total sum would establish the 5500 (=4990 + 510) years sought. The time God gave Adam to come to this World. The Ethiopian Orthodox Tewahedo Church based its calendar on this calculation.