Why 13 days apart specifically, and not a fixed number forever?

The gap grows by about 1 day every century that's a Gregorian non-leap year but a Julian leap year — it was 10 days at the 1582 reform, is 13 days now, and will become 14 days in the year 2100.

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Julian ↔ Gregorian Date Converter

Convert between the Julian and Gregorian calendars, still 13 days apart today.

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About the Julian ↔ Gregorian Date Converter

Converts dates between the modern Gregorian calendar and the older Julian calendar, still used by several Orthodox churches for religious calculations.

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How to use it

Choose a direction: Gregorian to Julian, or Julian to Gregorian.
Enter the date.
Read the converted date.

Formula

Exact Julian Day Number arithmetic (Fliegel & Van Flandern-style integer algorithms) in both directions — not a fixed offset, which is only valid for a specific century range and silently drifts wrong outside it.

Worked example

September 7, 2026 (Gregorian) converts to August 25, 2026 in the Julian calendar.

Interpreting your result

The Julian and Gregorian calendars have drifted 13 days apart since the Gregorian reform of 1582, because the Julian leap-year rule (every 4 years, no exceptions) adds 3 extra days every 400 years compared to the Gregorian rule (which skips century years not divisible by 400). That gap grows by roughly 1 day per skipped century.

Recommendations

• This is a general calendar utility, most relevant for understanding Orthodox church calendar dates — see this site's Orthodox Easter Calculator for the specific Paschal application.

Frequently asked questions

Several Orthodox churches (Russian, Serbian, Jerusalem, and others) use it for religious/liturgical calculations, even though their countries use the Gregorian calendar civilly. It's also still used by some Amazigh/Berber communities for agricultural purposes.