Anno Domini
Anno Domini (Latin for "In the Year of the Lord"), abbreviated as AD, defines an epoch based on the traditionally-reckoned year of the 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.
Many non-Christians use the abbreviations AD and BC without intending to acknowledge the Christian connotation. Some people prefer the alternatives CE (usually "Common Era") and BCE ("Before Common Era"), arguing that they are religiously neutral terms. (See the Common Era article for other possible interpretations of CE and BCE.)
Apologetics
Christians sometimes claim that because it uses the terms "BC" and "AD" the Gregorian calendar proves that Jesus existed.
