How many days apart are the Gregorian and Julian calendars?

The gap grows over time because the two calendars use different leap year rules for century years. It was 10 days in the 16th century when the Gregorian reform began and has since grown to 13 days in the 20th and 21st centuries.

Why do the calendars drift apart at all?

The Julian calendar treats every 4th year as a leap year with no exception, while the Gregorian calendar skips the leap day in century years not divisible by 400 (like 1700, 1800, 1900). Those skipped Julian leap days accumulate into the growing gap.

Is this the same thing as a Julian Day Number?

No, despite the similar name, the Julian calendar is a dated calendar system with months and years, while a Julian Day Number is a separate continuous day count used mainly in astronomy. This tool converts between two calendars; use Convert a Date to a Julian Day for the day-count version.

Does this model real historical adoption dates?

No, it applies the proleptic (mathematically extended) Julian and Gregorian rules uniformly across all dates. Real-world adoption of the Gregorian calendar happened on different dates in different countries, from 1582 in parts of Europe to the 20th century elsewhere.

Gregorian to Julian Calendar Converter - Convert Modern Dates to Old Style

Overview

Introduction

Before the Gregorian reform of 1582, the Western world used the Julian calendar, and the two systems drift further apart with every century that isn't a multiple of 400.

This tool converts a modern Gregorian date into its Julian calendar equivalent, applying the exact leap year rules of both calendars rather than a fixed day offset.

What Is Convert Gregorian Calendar to Julian Calendar?

A Gregorian-to-Julian calendar date converter, distinct from a Julian Day Number converter despite the shared name "Julian".

The Julian calendar, introduced by Julius Caesar in 46 BCE, uses a simple every-4-years leap year rule; the Gregorian calendar refined that rule in 1582 by dropping the leap day in most century years, which is why the two calendars slowly diverge.

How Convert Gregorian Calendar to Julian Calendar Works

The input Gregorian date is first converted into a Julian Day Number, a continuous day count that both calendars can map to and from.

That Julian Day Number is then converted back into a calendar date using the Julian calendar's own leap year rule, producing the equivalent Julian calendar date.

When To Use Convert Gregorian Calendar to Julian Calendar

Use it when working with historical records, genealogy, or older documents that record dates in the Julian ("Old Style") calendar, or when you need to express a modern date in Julian calendar terms for historical comparison.

It's also useful for understanding just how far the two calendars have drifted apart by a given year.

Often used alongside Convert Julian Calendar to Gregorian Calendar, Convert a Date to a Julian Day and Convert a Julian Day to a Date.

Features

Advantages

Applies the exact leap year rules of both calendars via a shared Julian Day Number, rather than a rough fixed-offset approximation.
Correctly reflects the growing gap between the calendars across different centuries.
Instant, no manual day-offset lookup required.

Limitations

Uses the proleptic (mathematically extended) versions of both calendars uniformly, it doesn't model real-world, country-by-country Gregorian adoption dates.
Does not account for the 1582 Gregorian reform's one-time skip of 10 calendar days, since it treats both calendars as continuously extended systems rather than a single historical timeline.

Examples

Converting a modern date

Input

2026-08-05

Output

2026-07-23

As of the 21st century, the Julian calendar trails the Gregorian calendar by 13 days.

Best Practices & Notes

Best Practices

Remember that the offset between the calendars is not fixed, it grows by roughly 3 days every 400 years, so don't reuse a day-offset from one century for another.
Use Convert Julian Calendar to Gregorian Calendar to reverse the conversion and confirm a result.

Developer Notes

The conversion routes through the shared Julian day number algorithm in julian-day-math.ts: `gregorianToJulianDayNumber` (the Fliegel & Van Flandern algorithm) converts the input date to a JDN, then `julianDayNumberToCalendarDate` with `gregorian=false` converts that same JDN back to a Julian calendar date using Richards' algorithm, so both calendars' leap year rules are applied exactly rather than approximated.

Convert Gregorian Calendar to Julian Calendar Use Cases

Cross-referencing dates in historical documents that use Old Style (Julian) dating
Genealogy research involving pre-Gregorian-reform records
Understanding the accumulated day offset between the two calendars for a given era

Common Mistakes

Assuming a fixed 13-day (or 10-day) offset applies to all eras, the offset changes depending on which century is involved.
Confusing this calendar conversion with a Julian Day Number lookup, which is a different, unrelated day-counting system despite the shared name.

Tips

Pair this with Convert Julian Calendar to Gregorian Calendar to round-trip and verify a conversion.
For a continuous day-count instead of a calendar date, use Convert a Date to a Julian Day.

Convert Gregorian Calendar to Julian Calendar