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.

Gregorian ↔ Julian Calendar Converter

Historical documents recorded dates in the Julian (old-style) calendar long after astronomers and most of Europe switched to the Gregorian (new-style) calendar.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.

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 — about 13 days for any date in the 20th and 21st centuries.

Example and tips

Converting Gregorian 1700-02-28 to Julian gives 1700-02-18, a 10-day gap, since 1700 was a Julian leap year but not a Gregorian one. The gap widened to 11 days after 1700, 12 after 1800, and 13 after 1900. 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.