Over time, ancient civilizations calculated and wrote about the passing of time differently than we do today. In Mesopotamia and Egypt, they based the calendar upon the king, or the seasons set by their various gods. In Rome, time was counted from the founding of Rome and changed periodically by rulers. In Mesoamerica, the Aztec Calendar (1.8) was the system used by the Pre-Columbian people, a 365-day calendar defining a century as 52 years long and based on the sun, a sacred symbol. Since time was established thousands of years ago by many different cultures, one system was not in use.
In the 6th century, Dionysius Exiguus, who was a Christian monk, established the Anno Domino (AD) and Before Christ (BC) as the reference date for the year zero in Europe based on the tenets of Christianity. Other religions also developed their calendars, and some are still in use today. The scholarly alternative to the current Christian designations for time is named Before the Common Era (BCE) and Common Era (CE) and has been adopted by academic and scientific publications and studies to emphasize secularism and inclusiveness. The new designation removed the specific religious designation from the calendar; instead, the new naming convention is more meaningful across the globe.
Scholars have readily adopted the new BCE/CE designation for communication and modernizing a worldwide standard. Many cultures today use a dual calendar designation, the BCE/CE standard, and their historical calendars. This textbook uses BCE and CE as a contemporary designation for all cultures around the world. For example, if art were discussed from Mesopotamia 5,000 years ago, it would state "in Mesopotamia, 3,000 BCE…". If discussing Gothic art, it would state "Gothic art, 1342 CE, the architectural style…". Using BCE for all dates to the year zero, and CE for all the dates after year zero is a simple clarification.
All dates, regardless of calendars, are based upon estimations since no one is sure when the year zero started. We are into the 2020th year now and cannot change the system to begin at a new date, and it would cause chaos in the computer systems. Year 2K was enough of a coding problem just moving from the 1900s to 2000, let alone moving the world to a new date.