In ancient times, there were two main types of calendar: the lunar calendar and the solar calendar. The lunar calendar was based on the phases of the moon, and the solar calendar was based on the position of the sun. The lunar calendar was used in most cultures until the introduction of the Gregorian calendar in 1582.

The lunar calendar is still used in some cultures today. The Islamic calendar is a lunar calendar, and the Chinese calendar is also lunar-based.

The terms 'AC' and 'BC' are often used to designate dates on the Gregorian calendar. 'AC' stands for 'Anno Domini' which means 'in the year of our Lord.' 'BC' stands for 'Before Christ.' Therefore, 'AC' dates are always later than 'BC' dates.

For example, the year 2018 is 'AC' because it is after the birth of Christ. The year 1 BC is 'BC' because it is before the birth of Christ.

AC and BC are terms often used to designate specific periods in history. However, the meaning of these terms has changed over time, and there is no single, universally accepted definition of them. In general, though, AC refers to the period before the Common Era (that is, before the year 1 CE), while BC corresponds to the period before Christ (that is, before the year 1 AD).

The use of these terms is not without controversy, as some people argue that they are Eurocentric and thus exclude other cultures and civilizations that did not use the Gregorian calendar. Additionally, the terms imply a linear view of history, which some scholars argue is inaccurate. Nevertheless, these terms are still widely used, and understanding their meaning is essential to understanding the historical periods they refer to.

The term 'AC' is sometimes used to refer to the period of time before the Common Era. This period includes all of history up to the year 1 AD. The term 'BC' is sometimes used to refer to the period of time before the Common Era. This period includes all of history up to the year 1 BC.

The terms 'AC' and 'BC' are also sometimes used to refer to the years before and after a specific event. For example, the term 'AC' may be used to refer to the years before the American Revolution, and the term 'BC' may be used to refer to the years after the American Revolution.