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Actually besides the BC, AD system, there are many other systems...
Anno Domini (Latin: "In the year of (Our) Lord"), abbreviated as AD, defines an epoch based on the traditionally-reckoned year of the conception or birth of Jesus of Nazareth. Similarly, Before Christ (from the Ancient Greek "Christos" or "Anointed One", referring to Jesus), abbreviated as BC, is used in the English language to denote years before the start of this epoch. Some non-Christians use the abbreviations AD and BC without intending to acknowledge the Christian connotation. Some people prefer the alternatives 'CE' and 'BCE', arguing that they are more neutral terms (see below).
The designation is used to number years in the Christian Era, conventionally used with the Julian and Gregorian calendars. More fully, years may be also specified as Anno Domini Nostri Iesu Christi ("In the Year of Our Lord Jesus Christ"). 'Anno Domini' dating was first calculated in 525 and began to be adopted in Western Europe during the eighth century.
The numbering of years per the Christian era is currently dominant in many places around the world, in both commercial and scientific use. For decades, it has been the global standard, recognized by international institutions such as the United Nations and the Universal Postal Union. This is due to the prevalence of Christianity in the Western world, the great influence of the Western world on science, technology and commerce, as well as the fact that the solar Gregorian calendar has, for a long time, been considered to be astronomically correct.
Traditionally English copies Latin usage by placing the abbreviation before the year number for AD, but after the year number for BC; for example: 64 BC, but AD 2006. However, the placing of the AD after the year number (as in 2006 AD) is now also common, while, analogous to the use of BC, the abbreviation is also widely used after the number of a century or millennium, as in 4th century AD or 2nd millennium AD, despite the inappropriate literal combination in this case ("in the 4th century in the year of Our Lord").資料來源： wiki