The Gregorian calendar is the most widely used civil calendar in the world today. For decades, it has been the unofficial global standard, adopted in the pragmatic interests of international communication, transportation, and commercial integration, and recognized by international institutions such as the United Nations. The terms anno Domini (AD) and before Christ (BC) are used to label or number years in the Julian and Gregorian calendars. The term anno Domini is Medieval Latin and means ‘in the year of our Lord’.
Countries which do not use the Gregorian calendar are Afghanistan, Iran, and Ethiopia. Some countries use other calendars alongside the Gregorian calendar: Bangladesh (Bangla calendar), India (Indian national calendar) and Israel (Hebrew calendar). Other countries use modified versions of the Gregorian calendar: Taiwan (Minguo calendar), Thailand (Thai solar calendar), North Korea (North Korean Calendar) and Japan (Japanese calendar).

There are a few things about the Gregorian calendar that might come as a surprise. One of the most controversial is the eleven days that were skipped over as part of the parliamentary measure that implemented the Gregorian calendar, aligning Britain and its overseas possessions with the rest of Western Europe— so the date September 3, 1752, to September 13, 1752, didn’t exist.

Calendars fall into four types, lunisolar, solar, lunar, seasonal, besides calendars with “years” of fixed length, with no intercalation. Most pre-modern calendars are lunisolar. The seasonal calendars rely on changes in the environment rather than lunar or solar observations. The Islamic and some Buddhist calendars are lunar, while most modern calendars are solar, based on either the Julian or the Gregorian calendars.

A solar calendar is a calendar whose dates indicate the position of Earth on its revolution around the Sun or, equivalently, the apparent position of the sun moving on the celestial sphere. The other commonly used system is a lunar calendar which dates the months based on cycles of the lunar phases. The Gregorian calendar is an example of the solar calendar.