Julian Date Converter – Convert Gregorian Date to Julian Date

Julian Date Converter Julian Date Converter

Calculated kyureki conversion

Japanese Old Calendar Converter

Convert Gregorian dates into a calculated Japanese old calendar date using a Japan UTC+9 lunisolar basis, or convert explicit old-calendar input back into Gregorian dates.

Enter a Gregorian date in YYYY-MM-DD format to convert it into a calculated Japanese old calendar date.

Live output

Result

Ready to convert

Choose a direction, enter a Gregorian date or explicit Japanese old calendar input, and the result appears here without leaving the page.

Guide

Convert dates with the Japanese Old Calendar Converter

The Japanese Old Calendar Converter changes Gregorian dates into Japanese old calendar dates and converts explicit Japanese old calendar input back into Gregorian dates. It is designed for deterministic date lookup where leap months and regional calendar basis need to be visible.

The page uses a calculated kyureki-style East Asian lunisolar calculation using Japan UTC+9. Reverse input uses OLD_YEAR-MONTH-DAY with optional -L for leap months so the converter can validate the exact year, month, day, and leap-month state instead of guessing from a festival name or cyclic year label.

This is a modern calculated old-calendar basis. It is not ICU Japanese era conversion and not an exhaustive historical almanac table.

What it handles

Japanese old calendar conversion coverage

The converter supports both Gregorian to Old Calendar and Old Calendar to Gregorian. Both directions use the same calendar basis, which makes the examples and reverse validation consistent.

Leap months are represented explicitly. If a leap-month marker is supplied for a date that the implementation does not produce, the converter rejects the input rather than silently changing the date.

Gregorian to Old Calendar

Enter a Gregorian date and return the matching Japanese old calendar date with leap-month state and weekday.

Old Calendar to Gregorian

Enter OLD_YEAR-MONTH-DAY with optional -L for leap months for a Japanese old calendar date and convert it back into Gregorian form.

Round-trip validation

The reverse direction checks that the converted date returns to the same calendar fields, including leap-month markers.

How to use

Use the Japanese old calendar conversion modes

Choose the mode that matches the source date you already have. The Gregorian direction accepts a standard date field; the reverse direction expects the explicit numeric calendar syntax shown in the form help.

Do not omit leap-month suffixes when the source date is in a leap month. Without the suffix, the converter treats the input as the regular month of the same number.

1

Choose Gregorian to Old Calendar

Use this direction for Gregorian input such as 2026-06-30. The verified example returns 2026-06-30 -> 2026-05-16.

2

Choose Old Calendar to Gregorian

Use this direction for explicit input such as 2026-05-16. Add the documented leap-month suffix only when the source date requires it.

3

Check the calendar basis

Use this page for the calculated Japanese old calendar basis. Japanese era-name conversion and historical local almanacs are outside this converter.

Examples

Reference conversions from the local implementation

These examples are generated from the same local conversion functions used by the form. They show the exact reverse-input notation accepted by this page and the Gregorian date returned when the value is converted back.

Gregorian date to Japanese old calendar date

2026-06-30 -> 2026-05-16

The Gregorian date 2026-06-30 converts to Japanese old calendar 2026-05-16.

Japanese old calendar date to Gregorian date

2026-05-16 -> 2026-06-30

The input 2026-05-16 converts back to Gregorian 2026-06-30.

FAQ

Japanese old calendar conversion notes

Traditional lunisolar calendars can look similar while using different time zones, calendar identifiers, or historical assumptions. These answers define what this page actually converts.

Can I enter a festival name or animal year?

No. Reverse conversion requires numeric year, month, and day fields because named years and festivals are not unique enough for deterministic conversion.

How are leap months handled?

Use the documented leap suffix only when the source date is in a leap month. The converter validates that the leap marker is possible for the entered date.

Does this replace a historical almanac?

This is a modern calculated old-calendar basis. It is not ICU Japanese era conversion and not an exhaustive historical almanac table.

Reference

Calendar implementation reference

This converter is documented against its implementation basis: a calculated kyureki-style East Asian lunisolar calculation using Japan UTC+9. The reference link points to ICU calendar services when the page uses an ICU calendar identifier.