<P> Alternative names for the Anno Domini era include vulgaris aerae (found 1615 in Latin), "Vulgar Era" (in English, as early as 1635), "Christian Era" (in English, in 1652), "Common Era" (in English, 1708), and "Current Era". Since 1856, the alternative abbreviations CE and BCE, (sometimes written C.E. and B.C.E.) are sometimes used in place of AD and BC . </P> <P> The "Common / Current Era" ("CE") terminology is often preferred by those who desire a term that does not explicitly make religious references . For example, Cunningham and Starr (1998) write that "B.C.E. / C.E....do not presuppose faith in Christ and hence are more appropriate for interfaith dialog than the conventional B.C. / A.D." Upon its foundation, the Republic of China adopted the Minguo Era, but used the Western calendar for international purposes . The translated term was 西元 ("xī yuán", "Western Era"). Later, in 1949, the People's Republic of China adopted 公元 (gōngyuán, "Common Era") for all purposes domestic and foreign . </P> <P> In the AD year numbering system, whether applied to the Julian or Gregorian calendars, AD 1 is preceded by 1 BC . There is no year "0" between them, so a new century begins in a year which has "01" as the final digits (e.g., 1801, 1901, 2001). New millennia likewise are considered to have begun in 1001 and 2001 . This is at odds with the much more common conception that centuries and millennia begin when the trailing digits are zeroes (1800, 1900, 2000, etc .); for example, the worldwide celebration of the new millennium took place on New Year's Eve 1999, when the year number ticked over to 2000 . </P> <P> For computational reasons, astronomical year numbering and the ISO 8601 standard designate years so that AD 1 = year 1, 1 BC = year 0, 2 BC = year − 1, etc . In common usage, ancient dates are expressed in the Julian calendar, but ISO 8601 uses the Gregorian calendar and astronomers may use a variety of time scales depending on the application . Thus dates using the year 0 or negative years may require further investigation before being converted to BC or AD . </P>

Why did the calendar change from bc to ad