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.

Calendar System Reference

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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.

Example

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.

Notes

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.