For the first five centuries of their religion, Christians marked time according to local conventions, usually from the legendary foundation of Rome (753 BC), or from the Diocletian reforms (284 AD).
In a sixth-century treatise on the calculation of Easter, Dionysius 'the Little' first proposed to count from the birth of Christ to avoid honouring the hated persecutor Diocletian.
But it is the Christian era, counting 'the years of the Lord' from the birth of Christ, that is now ubiquitous in business, politics and historical writing.
Note that this system is not generally used by Imperial citizens in everyday life, but is simply for administrative use by the Adeptus Terra.
This is the year within the millennium, running from 001-999. 2010 would be represented very generally in the Imperial Dating System as 010. As almost all records of the religions practiced by Mankind before the development of the Imperial Cult in the 31st Millennium have been lost, none know exactly why the Imperial Calendar counts the years in this manner or the true origin of the dating system.
Meanwhile, in 1615 Johannes Kepler used the phrase anno aerae nostrae vulgaris (in the year of our common era) in an astronomical table and 'Common Era' or its equivalents are known, if rare, in 18th-century works such as the Encyclopaedia Britannica of 1797.
Since the Christian and the secular chronologies yield identical dates (ad 2009 equals 2009 CE) why convert from one to the other?
Henceforth we have lived in the age of Christ's working in history: the years of Our Lord' - Anno Domini.