All history happens within time, and understanding time accurately is fundamental for historians. To describe when events took place and how long periods lasted, people developed systems of time measurement, especially calendars.
The calendar is one of humanity’s most important inventions.
Early farming societies needed it to plan sowing and harvesting.
Creating a reliable calendar required both mathematical knowledge and careful observation of nature.
Over time, three main types of calendars evolved:
- Lunar calendars, based on the phases of the Moon.
- Solar calendars, determined by the Earth’s orbit around the Sun.
- Lunisolar calendars, combining both cycles.
The ancient Julian calendar, introduced by Julius Caesar, was later reformed by Pope Gregory XIII in 1582 to create the Gregorian calendar, which is still used in most of the world today.
Our modern time system uses natural cycles:
- A day corresponds to one full rotation of the Earth.
- A month is roughly the time it takes the Moon to orbit the Earth.
- A year is the time the Earth needs to orbit the Sun.
To fix specific dates, a reference point is needed.
In the Western world, that point is the birth of Jesus Christ, marking the start of the Christian era.
Years are labeled either as A.D. (after Christ) or B.C. (before Christ).
Examples:
- The 13th century A.D. covers 1201–1300.
- The 8th century B.C. covers 800–701.
- Rome was founded in 753 B.C.
- Luther’s Reformation began in 1517 A.D., within the 16th century.
Other cultures count time differently:
- The Jewish calendar starts with the biblical creation of the world (3761 B.C.).
- The Roman calendar began with the founding of Rome (753 B.C.).
- The Islamic calendar begins in 622 A.D., the year Muhammad migrated from Mecca to Medina.
A curious fact is that there is no “year zero” in our calendar.
This happened because medieval scholars who established the Christian dating system did not yet understand or use the number zero.