Why do the two calendars differ by a number of days?

The Julian calendar adds a leap day every 4 years, slightly overestimating the solar year. The Gregonian reform skips 3 leap days every 400 years to correct this drift. Over centuries the accumulated difference grows — about 10 days in 1582, 13 days from 1900 to 2099.

What is a Julian Day Number?

The Julian Day Number is a continuous count of days since a fixed epoch (noon on 1 January 4713 BC, proleptic Julian). Both calendar dates are converted to this single integer, which makes converting between calendars and computing weekdays straightforward and unambiguous.

What does proleptic mean here?

A proleptic calendar is applied to dates before it was historically adopted. This converter uses the proleptic Julian and proleptic Gregorian systems, so it gives mathematically consistent results even for dates before 1582, where no single official conversion existed.

How does it handle the 1582 reform gap?

It does not skip the historical 10-day jump that some countries applied in October 1582, because different countries reformed in different years. Instead it treats each calendar as continuous and reports the pure day difference, which is what you need for arithmetic.

Can I find the weekday of a historical event?

Yes. The weekday is derived from the Julian Day Number, so it is correct regardless of calendar. Enter the date as recorded — Julian for most pre-1700 European records — and read the weekday from the result.

What is the Gregorian ↔ Julian Calendar Converter?

Convert any date between the modern Gregorian calendar and the old-style Julian calendar via the Julian Day Number. Shows the converted date, the weekday, the day-count gap between the two systems, and the underlying JDN. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Gregorian ↔ Julian Calendar Converter

Name: Gregorian ↔ Julian Calendar Converter
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Historical documents recorded dates in the Julian (old-style) calendar long after astronomers and most of Europe switched to the Gregorian (new-style) calendar. This converter moves a date cleanly between the two systems by routing through the Julian Day Number, and reports the day gap and weekday as well.

The Julian/Gregorian divide — why it matters

The Julian calendar, introduced by Julius Caesar in 46 BC, added a leap year every four years. That is slightly too generous: the solar year is about 11 minutes shorter than 365.25 days, so the Julian calendar gained roughly one day every 128 years. By 1582 the drift had reached 10 days, which put Easter on the wrong date relative to the spring equinox. Pope Gregory XIII issued the reform that became the Gregorian calendar, skipping 3 leap years every 400 years and jumping 10 days in October 1582 to correct the existing drift.

Different countries adopted the reform at very different times. Catholic countries shifted in 1582; Britain and its colonies did not change until 1752 (skipping 11 days); Russia did not adopt the Gregorian calendar until 1918 (skipping 13 days). This is why historical research involving dates across countries or before the 18th century requires careful calendar conversion.

How it works

Both calendars are converted to a single integer, the Julian Day Number (JDN), using the standard Fliegel–Van Flandern algorithms, then back out into the target calendar:

Gregorian (y,m,d) ──► JDN ──► Julian (y,m,d)
Julian    (y,m,d) ──► JDN ──► Gregorian (y,m,d)
weekday = (JDN + 1) mod 7

Because the JDN is a continuous day count, the difference between the two calendar dates for the same JDN is exactly the accumulated leap-day drift. The gap grows by one day each century year that is a Julian leap year but not a Gregorian one:

Date range	Day gap (Julian ahead of Gregorian)
1582–1700	10 days
1700–1800	11 days
1800–1900	12 days
1900–2100	13 days

Worked example

Converting Gregorian 1700-02-28 to Julian gives 1700-02-18 — a 10-day gap, because 1700 was a Julian leap year (adding February 29) but not a Gregorian one. The very next day, Gregorian 1700-03-01, the gap widens to 11 days and stays there until the next divergence in 1800.

For a historical research use case: if a Russian document records an event on Julian 1917-10-25, converting to Gregorian yields 1917-11-7 — which is why the Russian October Revolution is often referred to in the West as the “November Revolution.”

Tips for historical research

Rule of thumb for records: Church registers, court documents, and personal correspondence in England before September 14, 1752 (OS) follow the Julian calendar. Treat the date as Julian and convert.
Year-start differences: Before 1752, England also started the new year on March 25 (Lady Day) rather than January 1. Dates between January 1 and March 24 in English documents may be written as “1706/7” to indicate both the old-style and new-style year.
Scotland and Continental Europe: Scotland adopted January 1 as the year start in 1600. Many Continental Catholic countries switched at the 1582 reform. Always note which country’s document you are reading.
Astronomical usage: Astronomers use the Julian Day Number as a continuous count for calculating time intervals without calendar confusion. This tool exposes the JDN so you can verify dates against published astronomical tables.

When reading old church or court records from before a country’s reform, treat the recorded date as Julian and convert to Gregorian to compare against modern dates.