Julian Date Converter
Historical calendar conversion
Julian Calendar Converter
Convert Gregorian dates into historical Julian calendar dates, or convert Julian calendar dates back into Gregorian dates inside the same focused workspace.
Live output
Result
Choose a direction, enter a Gregorian or Julian calendar date, and the result appears here without leaving the page.
Guide
Convert Julian calendar dates and Gregorian dates
This Julian Calendar Converter changes Gregorian calendar dates into historical Julian calendar dates and converts Julian calendar input back into Gregorian dates. It is useful when a source records dates in the Julian calendar but you need to compare them with modern Gregorian dates.
Gregorian input uses the date field, while Julian calendar input uses YYYY-MM-DD in the text field. The result also includes a reference Julian Day Number internally used by the conversion, but the main output remains a calendar date with a year, month, and day.
This page is not the same as the Julian Date Converter. Julian calendar dates are historical calendar dates with months and days; Julian Day Number and decimal Julian Date are astronomical day counts handled by the homepage converter.
What it handles
Julian calendar conversion coverage
The converter handles arithmetic conversion between proleptic Gregorian dates and Julian calendar dates. It uses a Julian Day Number as an internal bridge so both directions share the same day reference.
It does not model country-by-country adoption of the Gregorian calendar. That is important for historical research because the civil calendar in use on a given date depended on jurisdiction; this page provides the arithmetic calendar equivalent.
Gregorian to Julian calendar
Enter a Gregorian date and return the corresponding Julian calendar year, month, day, and reference Julian Day Number.
Julian calendar to Gregorian
Enter a Julian calendar date in YYYY-MM-DD format and convert it back into the proleptic Gregorian calendar.
Julian leap-year rules
The converter validates Julian calendar dates using the Julian rule that every fourth year is a leap year.
How to use
Use the Julian calendar conversion modes
Choose the mode that matches the date you already have. Gregorian to Julian is for a modern-style Gregorian date; Julian to Gregorian is for a historical Julian calendar date written with a year, month, and day.
If your source value is a plain number, it is probably not a Julian calendar date. Numeric JDN and decimal JD values belong in the separate Julian Date Converter, where the input is interpreted as an astronomical day count.
Choose Gregorian to Julian
Use this direction when you have a Gregorian date such as 2026-06-30 and need the equivalent Julian calendar date.
Choose Julian to Gregorian
Use this direction when you have a Julian calendar date such as 2026-06-17. Enter it as YYYY-MM-DD.
Use the astronomical converter for JDN or JD
If your input is a number such as 2461222 or 2461221.5, use the Julian Date Converter instead of this Julian calendar page.
Examples
Reference conversions from the local implementation
The examples below are calculated by the same arithmetic functions used by the form. They show the current 13-day offset for this modern date range and include the reference JDN used internally for the conversion.
Gregorian date to Julian calendar date
2026-06-30 -> 2026-06-17The Gregorian date 2026-06-30 converts to Julian calendar 2026-06-17 with reference JDN 2461222.
Julian calendar date to Gregorian date
2026-06-17 -> 2026-06-30The Julian calendar input 2026-06-17 converts back to Gregorian 2026-06-30.
FAQ
Julian calendar conversion notes
Julian calendar searches often mix three different ideas: historical Julian calendar dates, Julian Day Numbers, and decimal Julian Dates. These answers keep this page scoped to calendar-date conversion only.
Is a Julian calendar date the same as a Julian Date?
No. A Julian calendar date has a year, month, and day in the historical Julian calendar. A Julian Date or Julian Day Number is an astronomical numeric day count.
What input format is accepted for Julian calendar dates?
Use YYYY-MM-DD. The converter validates the month, day, and Julian leap-year rule before converting to Gregorian.
Does this use country-specific calendar adoption dates?
No. The converter performs arithmetic conversion between proleptic Gregorian dates and Julian calendar dates. It does not model local historical adoption rules.
Reference
Calendar conversion reference
The converter uses standard Julian Day Number arithmetic as the bridge between calendar systems. The reference below is included because Julian calendar conversion is frequently confused with astronomical Julian Date lookup.