In 808 CE, George Syncellus began work on a Chronicle intending to cover "Creation" until his own times. By 810 CE, Syncellus had written up to 285 CE when Diocletian became Emperor. Syncellus's incapacitation or demise in 810 CE meant that this is where his Chronicle ends. Before he died, however, he convinced his friend Theophanes to finish the Chronicle - which he did. Theophanes took Syncellus's apparently large collection of source material and extended it to 813 CE. How much Theophanes's extended Chronicle was based on Syncellus' source material is a matter of debate (Neville, 2018:56). How much editing or redaction Theophanes performed on Syncellus' work up to 285 CE may also be unknown.

George Syncellus seemed to have had an obsession with time. He believed that Christ's Incarnation at the beginning of A.M.a 5501 and his Resurrection in A.M.a 5534 both occurred on 25 March, the same day as the day of creation (Tuffin and Adler, 2002:xxix). Syncellus and his descendant Theophanes used an older method the count years - the Alexandrian Anno Mundi system - A.M.a - which had at its basis a search for the date of Creation and the date of Easter. The end result of this quest, however, may be significant chronological uncertainty in the work of Theophanes/Syncellus. Both present multiple time markers in the combined text which are frequently inconsistent or inaccurate.

Not much is known about Syncellus' life. His date and place of birth seem to be unknown. Because Syncellus mentions having personally visited various sites in Palestine, there is some speculation that he may have lived for some time in a monastery near Bethlehem before moving to Constantinople but this is not known for sure (Neville, 2018:57 and Tuffin and Adler, 2002:xxx). At some point in time he was appointed to the high ranking ecclesiastical position of syncellus (literally "cell-mate") to Tarasios the Patriarch of Constantinople. He started writing his Chronicle after he retired from this position although presumably he would have been gathering source material before he began writing.

Some of George Syncellus' sources include Julius Africanus (c.160-240 CE), Eusebius of Caesarea (c. 260/265-339 CE), and the monks Panodoros and Annianos, both early 5th century monks from Alexandria who developed the Alexandrian version of Anno Mundi reckoning. Syncellus, like Theophanes, was strongly opposed to the iconoclasm of his time and may have suffered due to his outspoken views on the subject.