Julian Calendar Converter

About this calculator

This converter translates any date between the Julian calendar (Old Style, often abbreviated O.S.) and the Gregorian calendar (New Style, N.S.), in either direction. The Julian calendar was introduced by Julius Caesar in 45 BCE and was the everyday calendar of the Western world until the Gregorian reform of 1582. It uses exactly the same twelve month names and the same month lengths as the Gregorian calendar — January through December — so the two look identical on the page. The only structural difference is the leap-year rule, and that single difference is why the calendars have slowly drifted apart. One important warning before you start: this tool handles the Julian *calendar* (Old Style dates), not the astronomical "Julian Day" number used by astronomers, and not the ordinal "Julian date" (day-of-year, such as day 045) used in manufacturing and IT. By default it shows today's civil date converted to its Julian (Old Style) equivalent.

How to read your results

The toggle at the top chooses the direction: "Gregorian → Julian" turns a modern civil (New Style) date into its Old Style equivalent — handy for historians reading pre-1752 English documents or for working out an Orthodox feast — and "Julian → Gregorian" does the reverse. The headline result is the converted date — its year, named month, and day in the target calendar — followed by the day of the week, which is the same real weekday in both calendars because they describe the same 24-hour civil day. The result also shows the Old/New Style offset for that date: the number of days the Julian date trails the Gregorian one. Today that offset is 13 days, but it is not fixed — it grew from 10 days in 1582 and will become 14 days in the year 2100.

How it's calculated

Both directions pivot on the Julian Day Number (JDN), a continuous integer count of days that is independent of any calendar — confusingly named after Julius Scaliger, not after the Julian calendar. To convert, the input date is first reduced to its JDN, then that JDN is read out in the target calendar. The Gregorian side uses the standard civil-date formula; the Julian side uses the classic algorithm in which a year is a leap year whenever it is divisible by 4, with no centurial exception (note the Julian calendar has no year 0, so 1 BCE precedes 1 CE directly). Both algorithms come from John Walker's Fourmilab Calendar Converter, the same engine the rest of this site's calendar cluster is built on and cross-checked against. The day of the week is derived directly from the JDN, so it is identical for the two calendars. The Old/New Style offset shown with the result is computed as the difference in days between the source date and the same year/month/day read in the other calendar — 10 days at the 1582 reform, 13 days for 1900–2099, and 14 days from 2100. "Today" is computed at local civil midnight in your time zone after the page loads, so the default date is correct wherever you are without baking a build-time date into the page.

Worked example

Convert 7 January 2025 CE (Gregorian) to the Julian (Old Style) calendar.

7 January 2025 (New Style) is 25 December 2024 in the Julian (Old Style) calendar — a Tuesday in both calendars, with a 13-day Old/New Style offset. That is exactly why Orthodox churches that still follow the Julian calendar celebrate Christmas (25 December O.S.) on 7 January in civil New Style terms. Converting the other way around, 1 January 2000 (New Style) comes out as 19 December 1999 (Old Style) — the same 13-day gap, confirmed by the shared calendar engine.

Frequently asked questions

What is the difference between Old Style (O.S.) and New Style (N.S.) dates?

Old Style means a date written in the Julian calendar, the system used across the West before the Gregorian reform; New Style means the same day written in the Gregorian calendar we use today. Historians attach "O.S." or "N.S." to a date precisely because the two calendars give different numbers for the same day. For example, the dates on English documents before 1752 are Old Style. This converter moves a date between the two systems in either direction.

Why do Orthodox churches celebrate Christmas on 7 January?

Several Eastern Orthodox churches — including the Russian, Serbian, Georgian and Jerusalem patriarchates — still keep their liturgical calendar on the Julian (Old Style) system. They celebrate the Nativity on 25 December, but 25 December in the Julian calendar currently falls on 7 January in the civil Gregorian calendar, because the Julian calendar runs 13 days behind. So the date never moved; only the civil calendar around it changed.

Why is the Julian calendar 13 days behind, and will that change?

The Julian calendar makes every 4th year a leap year with no exception. That gives an average year of 365.25 days — about 11 minutes too long compared with the true solar year. Those minutes accumulate into roughly one extra day every 128 years. The Gregorian reform of 1582 fixed this by dropping the leap day in centurial years not divisible by 400 (so 1700, 1800 and 1900 were not leap years, but 2000 was). The gap was 10 days in 1582 and is 13 days for the years 1900–2099. It will widen to 14 days on 1 March 2100, when the Gregorian calendar skips a leap day that the Julian calendar keeps.

Is this the Julian calendar, the astronomical Julian Day, or an ordinal "Julian date"?

This tool converts the Julian *calendar* — the Old Style calendar of Julius Caesar. It is a different thing from two similarly named concepts. The astronomical "Julian Day" (or Julian Day Number) is a continuous running count of days used by astronomers, with no months at all; we use it internally as the conversion pivot and display it, but it is not what you enter. The ordinal "Julian date" used in manufacturing, logistics and IT is simply the day-of-year (for example day 045 means 14 February). If you came here looking for either of those, this calendar converter is not the right tool.

How are the Julian and Gregorian leap-year rules different?

Julian: every year divisible by 4 is a leap year, full stop. Gregorian: a year divisible by 4 is a leap year, except a year divisible by 100 is not, unless it is also divisible by 400. So 1900 is a Julian leap year (and 29 February 1900 is a real Old Style date) but not a Gregorian one, while 2000 is a leap year in both. That is the entire structural difference between the two calendars; the month names and lengths are otherwise identical.

Sources

• en.wikipedia.org/wiki/Julian_calendar
• www.fourmilab.ch/documents/calendar

Reviewed by the YouCalc Team · Last reviewedJune 13, 2026