Why do the Ethiopian and Gregorian calendars differ? Probably because Ethiopia was well outside the Roman Empire and never part of it, hence the nation never adopted it.
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: 8). 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.
selamforethiopia.com/ethioC.htm