This julian to gregorian calendar converter lets you translate any historical date between the Julian calendar (in effect from 45 BC, following Julius Caesar's 46 BC decree, until national reforms in the 1500s–1900s) and the Gregorian calendar we use today. Enter a date, pick a direction, and get the exact equivalent date, weekday, and the day-offset that caused the shift.
Choose a direction — Julian to Gregorian or Gregorian to Julian — then enter the year, era (AD or BC), month, and day. The fields are prefilled with October 4, 1582 AD, the date of Pope Gregory XIII's famous reform, so you can click "Convert Date" right away and see the tool in action. The result shows the equivalent date in the other calendar, the day of the week, and how many days apart the two calendars were at that point in history.
Historians, genealogists, and archivists constantly run into dates recorded in the "Old Style" Julian calendar — parish records, ship logs, treaties, and letters written before a country switched to the Gregorian system. Genealogists researching English, Russian, or Ottoman records regularly hit this problem, since Britain converted in 1752, Russia in 1918, and Greece as late as 1923. Astronomers and classicists also rely on precise calendar conversion when dating ancient eclipses, comet sightings, or manuscript colophons. This tool applies the day-offset relationship between the two calendars for any year, positive or BC. One caveat worth knowing: some Old Style British and colonial sources also began their calendar year on March 25 ("Lady Day") rather than January 1, which is why dates between January and March in that era are often written "dual dated" (for example, "10 February 1731/2"). This converter shifts the day and month accurately but does not reassign the year for that older new-year convention, so always check which convention your source uses before treating January–March dates as belonging to the following year.
The converter first turns your input date into a Julian Day Number (JDN) — a continuous day count used by astronomers since Joseph Scaliger introduced it in 1583. For a Julian calendar date it uses:
JDN = D + floor((153m+2)/5) + 365y + floor(y/4) − 32083, where y and m are adjusted so March is treated as the first month.
For a Gregorian calendar date, the same formula gets two extra correction terms for the century leap-year rule:
JDN = D + floor((153m+2)/5) + 365y + floor(y/4) − floor(y/100) + floor(y/400) − 32045
The JDN is then converted back into a calendar date using the inverse of whichever formula matches the target system. The day-of-week is found with (JDN + 1) mod 7, and the "offset" shown is the difference, in days, between how the two calendars label the same physical day.
Simplification note: this calculator assumes a perfectly regular Julian leap year every 4 years, going all the way back to year 1. In reality, Roman priests misapplied the rule between 45 BC and 8 AD, inserting leap days every 3 years instead of every 4, an error Augustus later corrected. Historians conventionally treat the Julian calendar as regularized from 8 AD onward, and — like most conversion tools — this calculator applies that same regular 4-year cycle uniformly, even for the handful of irregular decades before 8 AD.
Pope Gregory XIII decreed that Thursday, October 4, 1582 (Julian) would be immediately followed by Friday, October 15, 1582 (Gregorian) — deleting 10 days to correct centuries of drift. Enter Year 1582, Month October, Day 4, direction Julian → Gregorian, and the tool returns October 14, 1582 as the proleptic Gregorian equivalent (one day before the actual reform date, since the reform itself jumped an extra day). The offset shown is 10 days, matching the historical record exactly.
Julius Caesar decreed the calendar reform in 46 BC, guided by the astronomer Sosigenes, with the new Julian calendar taking effect starting 45 BC — fixing the year at 365.25 days with a leap day every four years. This was remarkably accurate for its time, but the true solar year is about 11 minutes shorter than 365.25 days — so the calendar drifted roughly one day every 128 years. By the 1500s, the spring equinox had shifted 10 days from where the Nicene Council of 325 AD had fixed it, throwing off the calculation of Easter. Pope Gregory XIII commissioned astronomer Aloysius Lilius to fix this, dropping 10 days in October 1582 and introducing the century leap-year exception still used worldwide today.
Catholic countries like Spain, Portugal, and Italy adopted the Gregorian calendar immediately in 1582, but Protestant England held out until 1752, by which point 11 days had to be dropped, sparking riots with crowds reportedly shouting "Give us our eleven days!" Russia didn't switch until after the 1917 Revolution, meaning the "October Revolution" actually took place in November by the Gregorian calendar. The Eastern Orthodox Church still uses the Julian calendar for religious feasts, which is why Orthodox Christmas falls on January 7 in the Gregorian calendar.
| Period | Days Gregorian is ahead of Julian |
|---|---|
| Up to 1699 | 10 |
| 1700–1799 | 11 |
| 1800–1899 | 12 |
| 1900–2099 | 13 |
| 2100–2199 | 14 |
That 13-day gap today is roughly equivalent to the calendar "losing" almost two full work-weeks compared to the sun's real position — the exact drift the Gregorian reform was designed to stop from ever growing further out of control.
The Julian calendar adds a leap day every 4 years with no exceptions, giving an average year of 365.25 days. The Gregorian calendar skips leap years in century years not divisible by 400 (like 1700, 1800, 1900), giving a more accurate average of 365.2425 days.
It depends on the century: 10 days for dates up to 1699, 11 days for the 1700s, 12 days for the 1800s, and 13 days for 1900 through 2099. This tool calculates the exact offset automatically for any year.
The Gregorian reform was issued by the Catholic Pope Gregory XIII in 1582, so Catholic nations adopted it quickly while Protestant and Orthodox nations were slower to accept a papal decree. Britain waited until 1752 and Russia until 1918.
It means applying Gregorian calendar rules to dates before the calendar actually existed (before 1582), purely as a mathematical extension. Historians use this to compare dates consistently across eras.
No — the traditional calendar goes directly from 1 BC to 1 AD with no year zero. This converter accounts for that when translating BC years into the astronomical numbering used in the underlying formulas.
Yes — use the "Gregorian → Julian" tab above, enter your modern date, and the tool will show you the equivalent Julian (Old Style) calendar date.
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