In order to be able to count the days, you need a starting point. For Christians using the Julian and Gregorian calendars this has been based on the traditionally agreed year of the birth of Jesus. AD counts years from the start of this time, and BC denotes the years before his birth. There is no year zero in this scheme, so the year AD 1 immediately follows the year 1 BC. Therefore, we are now in 2021 AD.
- BC (Before Christ), that is before the birth of Christ Jesus
- AD (Anno Domini), means ‘in the year of our Lord’ and beyond
Since the late 20th century, Common Era terms have become popular as culturally and religiously neutral terms replacing BC and AD. They start from the same point:
- BCE (Before Common Era) is the same as BC. So, 500 BC is the same as 500 BCE
- CE (Common Era) is the same as AD. So, 2021 AD is the same as 2021 CE
Not everyone uses this system. For instance the Hebrew year count starts in year 3761 BCE, which the 12th-century Jewish philosopher Maimonides established as the biblical Date of Creation. By this count we are now in year AM (Anno Mundi) 5782.