Leap year

A leap year (also known as an intercalary year or a bissextile year) is a year containing one additional day (or, in the case of lunisolar calendars, a month) added to keep the calendar year synchronized with the astronomical or seasonal year. Because seasons and astronomical events do not repeat in a whole number of days, calendars that have the same number of days in each year drift over time with respect to the event that the year is supposed to track. By inserting (also called intercalating) an additional day or month into the year, the drift can be corrected. A year that is not a leap year is called a common year.

For example, in the Gregorian calendar, each leap year has 366 days instead of the usual 365, by extending February to 29 days rather than the common 28. Similarly, in the lunisolar Hebrew calendar, Adar Aleph, a 13th lunar month, is added seven times every 19 years to the twelve lunar months in its common years to keep its calendar year from drifting through the seasons.

Gregorian calendar

The Gregorian calendar, also called the Western calendar and the Christian calendar, is internationally the most widely used civil calendar. It is named for Pope Gregory XIII, who introduced it in 1582.

The calendar was a refinement to the Julian calendar amounting to a 0.002% correction in the length of the year. The motivation for the reform was to bring the date for the celebration of Easter to the time of the year in which it was celebrated when it was introduced by the early Church. Because the celebration of Easter was tied to the spring equinox, the Roman Catholic Church considered the steady drift in the date of Easter caused by the year being slightly too long to be undesirable. The reform was adopted initially by the Catholic countries of Europe. Protestants and Eastern Orthodox countries continued to use the traditional Julian calendar and adopted the Gregorian reform after a time, for the sake of convenience in international trade. The last European country to adopt the reform was Greece, in 1923.

Common Era

Common Era or Current Era, abbreviated CE, is a calendar era that is often used as an alternative naming of the Anno Domini system ("in the year of the Lord"), abbreviated AD. The system uses BCE as an abbreviation for "before the Common (or Current) Era" and CE as an abbreviation for "Common Era". The CE/BCE designation uses the year-numbering system introduced by the 6th-century Christian monk Dionysius Exiguus, who started the Anno Domini designation, intending the beginning of the life of Jesus to be the reference date. Neither notation includes a year zero, and the two notations (CE/BCE and AD/BC) are numerically equivalent; thus "2016 CE" corresponds to "AD 2016", and "400 BCE" corresponds to "400 BC". The Gregorian calendar and the year-numbering system associated with it is the calendar system with most widespread use in the world today. For decades, it has been the global standard, recognized by international institutions such as the United Nations and the Universal Postal Union.

Anno Domini

The term anno Domini is Medieval Latin, which means in the year of the Lord but is often translated as in the year of our Lord. It is occasionally set out more fully as anno Domini nostri Iesu (or Jesu) Christi ("in the year of Our Lord Jesus Christ"). The terms anno Domini (AD or A.D.) and before Christ (BC or B.C.) are used to label or number years in the Julian and Gregorian calendars.

This calendar era is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth, with AD counting years from the start of this epoch, and BC denoting years before the start of the era. There is no year zero in this scheme, so the year AD 1 immediately follows the year 1 BC. This dating system was devised in 525, but was not widely used until after 800.

The Gregorian calendar is the most widely used 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.

2nd millennium

The second millennium is a period of time that began on January 1, 1001 and ended on December 31, 2000 of the Gregorian calendar. It was the second period of one thousand years in the Anno Domini or Common Era.

It encompassed the High and Late Middle Ages, the Mongol Empire, the Renaissance, the Baroque era, the early Modern Age, the age of Enlightenment, the age of colonialism, industrialization, the rise of nation states, and the 20th century with the impact of science, widespread education, and universal health care and vaccinations in many nations. The centuries of expanding large-scale warfare with high-tech weaponry (of the World Wars and nuclear bombs) were offset by growing peace movements, the United Nations, plus doctors and health workers crossing borders to treat injuries and disease, and the return of the Olympics as contest without combat.

Scientists prevailed in explaining intellectual freedom; humans took their first steps on the Moon during the 20th century; and new technology was developed by governments, industry, and academia across the world, with education shared by many international conferences and journals. The development of movable type, radio, television, and the internet spread information worldwide, within minutes, in audio, video, and print-image format to inform, educate and entertain billions of people by the end of the 20th century.