Julian / Gregorian Date Converter

Convert between Julian and Gregorian calendar dates, including Julian century leap days, using explicit proleptic calendar rules.

Starts with an editable example. Replace the dates and rules with your own.

Your calculation

Ready when you are

Choose your dates.

Select your counting rules and calculate.

How to use this calculator

Select a direction and enter the date in its original calendar. Both the input and converted date must be in years 1900-2100.

The tool extends each calendar's rules consistently and does not apply a country's historical switch date. Julian years divisible by four are leap years, including 1900 and 2100.

A worked example

Gregorian September 11, 2026 is Julian August 29, 2026.

Does it convert astronomical Julian Date numbers?

Use the separate Julian Date Converter for continuous JD or MJD day numbers. This tool converts calendar labels.

Which calendar and years are supported?

Each result states its calendar, time scale, precision and supported range. Batch converters accept up to 100 nonempty input lines and retain invalid rows. Copy, CSV and print include the calculation rules. Calendar downloads appear when a result contains dated events. Calculations run in your browser; no reminders or jobs are scheduled.

What happens to my dates?

Calculations run on your device. Saving a calculation is optional and keeps it in this browser. A setup link contains the dates and settings you entered; anyone with the link can read them. Use one tab per saved calculator. Clear saved data from Privacy & settings.