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system of organizing days for social, religious, commercial, or administrative purposes.

A calendar is a tool for organizing days. People use calendars to say when something happened, and for planning things that have not happened yet. The calendar gives the days names and numbers, called calendar dates. The dates are usually made based on how things in the sky seem to move. The year and month are based on motions of the Sun and moon. By knowing what day something happened or will happen, people have an easier life. Since the beginning of history, knowing when the seasons would start for crops was very important for farmers and people who eat.

2019 in various calendars
Gregorian calendar2019
Ab urbe condita2772
Armenian calendar1468
Assyrian calendar6769
Bahá'í calendar175–176
Balinese saka calendar1940–1941
Bengali calendar1426
Berber calendar2969
British Regnal year67 Eliz. 2 – 68 Eliz. 2
Buddhist calendar2563
Burmese calendar1381
Byzantine calendar7527–7528
Chinese calendar戊戌(Earth Dog)
4715 or 4655
    — to —
己亥年 (Earth Pig)
4716 or 4656
Coptic calendar1735–1736
Discordian calendar3185
Ethiopian calendar2011–2012
Hebrew calendar5779–5780
Hindu calendars
 - Vikram Samvat2075–2076
 - Shaka Samvat1940–1941
 - Kali Yuga5119–5120
Holocene calendar12019
Igbo calendar1019–1020
Iranian calendar1397–1398
Islamic calendar1440–1441
Japanese calendarHeisei 31 / Reiwa 1
Javanese calendar1952–1953
Juche calendar108
Julian calendarGregorian minus 13 days
Korean calendar4352
Minguo calendarROC 108
Nanakshahi calendar551
Thai solar calendar2562
Tibetan calendar阳土狗年
(male Earth-Dog)
2145 or 1764 or 992
    — to —
(female Earth-Pig)
2146 or 1765 or 993
Unix time1546300800 – 1577836799

Calendar systems have a beginning time or era. The calendar era is often a religion-based event, such as the birth of Jesus, but it may be a political event such as a coronation or the founding of a state. Dates that are counted from the coronation are called "regnal".

The three principal calendars most used today are the Gregorian, Hebrew, and Islamic calendars. Other calendar systems from many different parts of the world are also used.


Calendars are based on three natural things:

  1. The Earth turns all the way around its axis.
  2. The Moon goes all the way around the Earth.
  3. The Earth goes all the way around the Sun.

The Earth turns all the way around its axis about 24 hours. It is called the Solar Day. Days of most calendars are strongly based on the Solar Day.

The Moon turns all the way around the Earth about 29 days, 12 hours, 44 minutes and 2 seconds. Months of some calendars are still strongly based on the Moon. These are called Lunar calendars, which lasts about 354-355 days. Solar calendars ignore the moon, and depend entirely on the Sun.

The Earth goes all the way around the Sun in about 365 days, 5 hours, 48 minutes and 46 seconds. This is called the Tropical Year. Years of most calendars are strongly based on the Tropical Year.

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