calendar

Introduction
Measurement of time and types of calendars
- Standard units and cycles
- Time determination by stars, Sun, and Moon
- Complex cycles
Ancient and religious calendar systems
- The Near East and the Middle East
  - Babylonian calendars
  - Other calendars used in the ancient Near East
    - The Assyrians and the Hittites
    - Iran
  - The Egyptian calendar
  - Ancient Greek calendars in relation to the Middle East
    - Earliest sources
    - The year
    - Months, days, seasons
  - The early Roman calendar
  - The Jewish calendar
    - The calendar in Jewish history
    - The structure of the calendar
    - Months and important days
  - The Muslim calendar
- The Far East
  - The Hindu calendar
    - Early history
    - The classical calendar
    - The sacred calendar
    - The eras
  - The Chinese calendar
- The Americas
  - The Mayan calendar
  - The Mexican (Aztec) calendar
  - Peru: the Inca calendar
  - North American Indian time counts
The Western calendar and calendar reforms
- The Julian calendar
- The Gregorian calendar
  - The date of Easter
  - Adoption in various countries
- Calendar reform since the mid-18th century
  - The French republican calendar
  - Soviet calendar reforms
  - Modern schemes for reform

References & Edit History Related Topics

Images, Videos & Interactives

Kalendarium (“Calendar”) by Regiomontanus

Infographic showing date formats: Day-Month-Year (most of world), Year-Month-Day (China, Japan, etc.), and Month-Day-Year (United States).

Illuminated manuscript page by the Limbourg brothers showing a procession of nobles on horseback in elaborate medieval dress, with a blue astrological calendar arch above.

What Is Leap Day?

Interactive Gregorian calendar tool with fields for year, month, day, and era, set to December 8, 2025, and a blue Calculate button at the bottom.

Aerial view of Stonehenge stone circle with large upright and fallen stones arranged on green grass.

Circular diagram of the Jewish religious year showing Hebrew months around the outer ring and Gregorian months inside, with major holidays and fasts such as Rosh Hashanah, Yom Kippur, Sukkot, Hanukkah, Passover, and Purim labeled with their dates.

Late 18th-century Tibetan manuscript showing colorful illustrations of the twelve Chinese zodiac animals, each surrounded by handwritten script and calendar markings.

Circular illustration of the Chinese zodiac with animal symbols, including dragon, snake, horse, and tiger, arranged around a central yin-yang symbol on a purple patterned background.

Aztec calendar stone

At a Glance

Round metal perpetual calendar featuring a ship's wheel and a sailing ship, with rotating dials to align years and months for finding days of the week from 2003 to 2052.

calendar summary

Related Questions

Kalendarium (“Calendar”) by Regiomontanus — Kalendarium (“Calendar”) by Regiomontanus First complete printed title page for the Kalendarium (“Calendar”) by Regiomontanus, 1476.

calendar

chronology

Written by

John D. SchmidtAll

Fact-checked by

Britannica Editors

Last updated

Sep. 26, 2026 •History

calendar, any system for dividing time over extended periods, such as days, months, or years, and arranging such divisions in a definite order. A calendar is convenient for regulating civil life and religious observances and for historical and scientific purposes. The word is derived from the Latin calendarium, meaning “interest register” or “account book,” itself a derivation from calendae (or kalendae), the first day of the month in the Roman republican calendar, the day on...

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