Gregorian Calendar

The Gregorian calendar is a solar calendar, meaning its year is based on Earth's orbit around the sun. It is the most widely used dating system in the world, adopted by most governments, international organizations, and commercial entities.

The calendar year is divided into 12 months, totalling 365 days. Every fourth year, one extra day is added to February, making that year a leap year of 366 days. The purpose of the added day is to keep the calendar synchronized with the actual length of Earth's orbit around the sun, which is slightly more than 365 days.

Origins and reform

The Gregorian calendar derives from the Julian calendar that was used in Europe. The Julian calendar estimated the year as 365 days plus a quarter, which is slightly longer than the actual duration. Consequently, after many centuries, the calendar dates drifted from the actual seasons, such as the spring equinox. The reform known as the Gregorian reform was introduced in the sixteenth century, with a new leap-year rule: century years (such as 1700, 1800, 1900) are leap years only if divisible by 400, which is why 1600 and 2000 were leap years.

Gradual adoption

Countries did not adopt the new calendar all at once. Some Catholic countries adopted it early, while Protestant and Orthodox countries did so centuries later. For this reason, historical documents are sometimes written using both systems ("old style" and "new style"). Some Orthodox churches still reckon their religious observances according to the Julian calendar.

Use alongside other calendars

In Somalia and most Muslim-majority countries, the Gregorian calendar is used for government, education, and commerce, while the Islamic (lunar) calendar is used for religious occasions such as Ramadan, the Eid festivals, and the Hajj pilgrimage. Other calendars in use by specific peoples or faiths include the Hebrew, Chinese, and Ethiopian calendars.

The numbering of years in the Gregorian calendar begins from the date assigned to the birth of Jesus Christ, from which the term "Miilaadi" (birth) derives. In scholarly work, the designations CE (Common Era) and BCE are often used instead of AD and BC, with the same numerical values.