What is the difference between the Julian and Gregorian calendars?

Both are solar calendars with the same months, but the Julian calendar adds a leap day every 4 years without exception, giving an average year of 365.25 days. The Gregorian calendar skips the leap day in century years not divisible by 400, giving 365.2425 days, which tracks the solar year far more closely.

Why is the Islamic (Hijri) year shorter than the Gregorian year?

The Islamic calendar is purely lunar, with 12 lunar months totalling about 354 days, roughly 11 days shorter than the solar year. Because it does not intercalate to match the seasons, Islamic dates drift earlier through the Gregorian year, and the Hijri year number gains on the Gregorian over time.

What kind of calendar is the Hebrew calendar?

The Hebrew calendar is lunisolar: months follow the moon, but a 13th leap month is added in 7 of every 19 years to keep the calendar aligned with the solar year and seasons. Its epoch is the traditional date of creation, so its year number is far higher than the Gregorian.

Why does the Buddhist calendar year look so high?

The Thai Buddhist calendar counts years from the traditional date of the Buddha's passing, 543 years before the Common Era. So a Gregorian year of 2026 corresponds to roughly Buddhist Era 2569. The month structure otherwise follows the Gregorian calendar in modern Thai civil use.

How does this tool compute the other calendars?

It uses the browser's built-in Intl.DateTimeFormat with calendar options such as hebrew, islamic, persian, and buddhist. These are standard JavaScript internationalisation features, so the conversions are computed locally with no network request.

What is the Calendar System Reference?

Reference for the world's major calendar systems with epoch, year length, leap rule, and approximate current year, plus a live converter showing today's date in several calendars. It runs free in your browser on Gera Tools, with nothing uploaded.

Calendar System Reference

Name: Calendar System Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Humanity runs on several incompatible calendars at once, differing in epoch, year length, and how they keep step with the sun or moon. This reference summarises the major systems and shows today’s date rendered in each, using your browser’s built-in internationalisation engine.

How it works

Calendars fall into three families. Solar calendars (Gregorian, Julian, Persian) track the sun and keep months roughly aligned with the seasons. Lunar calendars (Islamic/Hijri) follow only the moon, so they drift through the seasons. Lunisolar calendars (Hebrew, traditional Chinese) follow the moon but insert leap months to stay aligned with the solar year.

Each calendar also has an epoch — a year-zero or year-one anchor — which is why the current year number varies so widely. The Hebrew epoch is the traditional creation date; the Islamic epoch is the Hijra; the Buddhist epoch precedes the Common Era by 543 years.

The major calendar systems at a glance

Calendar	Type	Epoch anchor	Leap rule
Gregorian	Solar	Year 1 CE	Divisible by 4, except centuries unless divisible by 400
Julian	Solar	Year 1 CE	Every 4 years, no exceptions
Hebrew	Lunisolar	Traditional creation (3761 BCE)	7 leap months in 19-year Metonic cycle
Islamic (Hijri)	Lunar	Hijra, 622 CE	11 leap years in 30-year cycle
Persian (Solar Hijri)	Solar	Hijra, 622 CE	Complex 2820-year intercalation
Buddhist (Thai)	Solar	Buddha’s parinirvana, ~543 BCE	Gregorian leap rule
Chinese	Lunisolar	Mythological Yellow Emperor	Metonic-based leap months

Why the year numbers differ so much

Because each calendar starts counting from a different event. The Islamic Hijri year is currently in the 14th century because it started counting from the Prophet Muhammad’s migration to Medina in 622 CE, and its 354-day year gains on the Gregorian over time. The Hebrew year is in the 58th century because it counts from a traditional creation date of 3761 BCE. The Buddhist year (used officially in Thailand and several Southeast Asian countries) is always 543 years ahead of the Gregorian because it starts from an estimated date of the Buddha’s passing.

Practical implications

International documents: many Middle Eastern countries use the Hijri calendar for government and legal purposes alongside or instead of the Gregorian. Some administrative dates in Saudi Arabia, for example, are quoted in Hijri, so converting accurately matters for contracts and visa validity.

Religious observance: dates like Eid al-Fitr, Rosh Hashanah, and Diwali are calculated from lunar or lunisolar calendars. Because these calendars do not lock step with the Gregorian, the equivalent Gregorian date shifts by up to a few weeks each year. The Islamic calendar shifts the most — about 11 days earlier per Gregorian year.

Software date handling: when storing dates that span calendar systems, always store in Gregorian/ISO 8601 and convert for display. Mixing stored representations causes ambiguous dates, particularly for historical records where the Julian and Gregorian calendars overlap (both were in use simultaneously between 1582 and the mid-18th century).

Technical note

The tool calls Intl.DateTimeFormat with a calendar option for each system. A formatter created with { calendar: "islamic" } renders today’s Gregorian date as its Hijri equivalent entirely in the browser, with no network request.

Leap rules differ sharply: the Julian calendar over-counts leap years and has drifted 13 days behind the Gregorian since 1582. The Gregorian 400-year rule (skip century leap years unless divisible by 400) keeps it accurate to about one day in 3,000 years. When storing dates, prefer the Gregorian/ISO 8601 form and convert for display only.