Ancient dating systems
In a sense, Anno Domini is simply an extension of this, counting years of the "reign" of Jesus Christ.If the Anno Domini system hadn't been adopted, the United States might refer to the current date as "December 13 of the sixth year of the Obama Presidency", while the British would describe it as "December 13 of the 62nd year of the reign of Elizabeth II".
Prior years were numbered to count backward to indicate the number of years an event had occurred “before Christ” or “B. E.," or "before common era." Before we talk about how and why the system was invented, let's get some historical context. 325, had decided that Easter would fall on the Sunday following the full moon that follows the spring equinox. 525, a monk named Dionysius Exiguus of Scythia Minor introduced the A. system, counting the years since the birth of Christ. Dionysius never said how he determined the date of Jesus' birth, but some authors theorize that he used current beliefs about cosmology, planetary conjunctions and the precession of equinoxes to calculate the date. Up until this point, Dionysius’ system had been widely used. D." stands for , Latin for “in the year of the lord,” and refers specifically to the birth of Jesus Christ. C." stands for "before Christ." In English, it is common for "A. In the early Middle Ages, the most important calculation, and thus one of the main motivations for the European study of mathematics, was the problem of when to celebrate Easter. Computus (Latin for computation) was the procedure for calculating this most important date, and the computations were set forth in documents known as Easter tables. Dionysius devised his system to replace the Diocletian system, named after the 51st emperor of Rome, who ruled from A.