What is the difference between Gregorian and Julian calendars?

The Julian calendar, introduced by Julius Caesar in 46 BC, assumes a year is exactly 365.25 days long. The Gregorian calendar, adopted in 1582, refines this to 365.2425 days by skipping leap years in century years not divisible by 400. This makes the Gregorian calendar about 0.002% more accurate.

How many days apart are the two calendars today?

In the 20th and 21st centuries (1900–2099), the Julian calendar is 13 days behind the Gregorian calendar. For example, if today is March 14 in the Gregorian calendar, it is March 1 in the Julian calendar.

When did most countries switch to the Gregorian calendar?

Catholic countries switched in 1582. Great Britain and its colonies switched in 1752. Russia switched in 1918, Greece in 1923. Some Eastern Orthodox churches still use the Julian calendar for liturgical purposes.

What do "Old Style" and "New Style" mean?

"Old Style" (O.S.) refers to dates in the Julian calendar; "New Style" (N.S.) refers to the Gregorian calendar. Historians often append these abbreviations when citing dates from the transition period to avoid ambiguity.

Does this tool work for dates before 1582?

Yes. The tool uses the proleptic Gregorian calendar (extending Gregorian rules backward) and the Julian calendar for all dates. Dates before October 15, 1582 predate the formal Gregorian reform, so both calendars are shown for reference.

What is the Julian Day Number?

The Julian Day Number (JDN) is a continuous count of days since January 1, 4713 BC (Julian). It is widely used in astronomy to avoid calendar ambiguity across long time spans. This tool uses JDN internally for accurate conversion.

Gregorian to Julian Calendar Converter

Enter a Gregorian date and instantly see the equivalent Julian calendar date plus the exact day offset.

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Summary

The Gregorian calendar, introduced by Pope Gregory XIII in 1582, corrected the drift in the older Julian calendar by skipping days and adjusting leap-year rules. The difference between the two systems grows by three days every 400 years — reaching 13 days in the 20th and 21st centuries. This tool lets you enter any Gregorian date and immediately see its Julian calendar equivalent along with the precise day offset.

How it works

Enter a date using the Gregorian calendar (the standard calendar in use today).
The tool calculates the Julian Day Number for that Gregorian date.
It then reverses the calculation using Julian calendar rules to find the matching Julian date.
The day difference (offset) between the two systems is displayed.
You can also enter a Julian date to convert back to Gregorian.

Use cases

Research historical events recorded in the Julian (Old Style) calendar.
Convert Orthodox Christian feast days to the civil Gregorian calendar.
Calculate dates for events before 1582 when the Gregorian calendar was adopted.
Understand the 13-day difference used in 20th-century Russian history.
Genealogy research involving documents from different calendar eras.
Academic or astronomical date verification for historical records.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu