The Anno Domini era was introduced in 525 by Scythian monk Dionysius Exiguus (c. 470 – c. 544), who used it to identify the years on his Easter table.
This practice began with the English cleric Bede (c. 672–735), who used AD years in his Historia ecclesiastica gentis Anglorum (731), popularizing the era.
Bede also used – only once – a term similar to the modern English term "before Christ", though the practice did not catch on for nearly a thousand years, when books by Denis Pétau treating calendar science gained popularity.
Bede continued this earlier tradition relative to the AD era.
The modern English term "before Christ" (BC) is only a rough equivalent, not a direct translation, of Bede's Latin phrase ante incarnationis dominicae tempus ("before the time of the lord's incarnation"), which was itself never abbreviated.
[citation needed] Neither the concept of nor a symbol for zero existed in the system of Roman numerals.
The Babylonian system of the BC era had used the idea of "nothingness" without considering it a number, and the Romans enumerated in much the same way.
The first extensive use (hundreds of times) of 'BC' occurred in Fasciculus Temporum by Werner Rolevinck in 1474, alongside years of the world (anno mundi).
But the discontinuity between 1 AD and 1 BC makes it cumbersome to compare ancient and modern dates.
This year numbering notation was introduced by the astronomer Jacques Cassini in 1740.
[11] In 1702, the French astronomer Philippe de La Hire labeled a year as Christum 0 and placed it at the end of the years labeled ante Christum (BC), and immediately before the years labeled post Christum (AD), on the mean motion pages in his Tabulæ Astronomicæ, thus adding the number designation 0 to Kepler's Christi.
Programming libraries may implement a year zero, an example being the Perl CPAN module DateTime.