Julian to Gregorian Date Converter — Old Style to New Style Calendar
Convert dates from the Julian calendar (Old Style, OS) to the Gregorian calendar (New Style, NS) and back. The Julian and Gregorian calendars differ by a growing number of days — 10 days at the 1582 Gregorian reform and 13 days in the 20th century. Furthermore, the Calendar Difference tab shows the exact Julian–Gregorian offset for every century from 200 CE to 2100 CE in a single reference table.
How to use the Julian to Gregorian Date Converter
The Julian–Gregorian difference by century
The Julian calendar has simpler leap year rules than the Gregorian — every year divisible by 4 is a leap year in the Julian system. Furthermore, this produces too many leap years over time, causing the Julian calendar to drift behind the Gregorian at a rate of approximately 3 days per 400 years.
| Period | Julian behind Gregorian by | Note |
|---|---|---|
| Before 1582 | 0–10 days | Julian calendar was the standard |
| 1582–1700 | 10 days | Gregorian reform — "10 days lost" |
| 1700–1800 | 11 days | 1700 was not a Gregorian leap year |
| 1800–1900 | 12 days | 1800 was not a Gregorian leap year |
| 1900–2100 | 13 days | 1900 was not a Gregorian leap year |
| From 2100 | 14 days | 2100 will not be a Gregorian leap year |
Countries that adopted Gregorian late
Different countries adopted the Gregorian calendar at very different times. Furthermore, England and its colonies (including the American colonies) switched in September 1752 — at which point the offset had grown to 11 days. Moreover, Russia switched in February 1918 after the Bolshevik Revolution — skipping 13 days. This means many historical dates in Russian documents before 1918 are in Julian (Old Style).
How Julian to Gregorian conversion works
The conversion uses Julian Day Numbers as an intermediary. Furthermore, both calendars express their dates as JDNs using different formulas — the Julian formula has no century correction terms, while the Gregorian formula subtracts century years not divisible by 400.
Conversion = Julian date → Julian JDN → Gregorian date from same JDN
Offset = Julian JDN − Gregorian JDN (for same nominal date) = days Julian is ahead
In 2025 = Julian calendar is 13 days behind Gregorian
Worked example: the Gregorian calendar reform of 1582
When Pope Gregory XIII introduced the Gregorian calendar in October 1582, 10 days were skipped. The day after Thursday 4 October 1582 (Julian) was Friday 15 October 1582 (Gregorian).
| Julian date | Gregorian equivalent | Offset |
|---|---|---|
| 4 October 1582 (Julian) | 14 October 1582 (Gregorian) | 10 days |
| 5 October 1582 (Julian) | 15 October 1582 (Gregorian) | 10 days (Gregorian reform began) |
| 25 December 1642 (Julian) | 4 January 1643 (Gregorian) | 10 days (Newton's birth) |
| 14 February 1918 (Julian) | 1 March 1918 (Gregorian) | 13 days (Russia switches) |
What is the Julian calendar?
The Julian calendar was introduced by Julius Caesar in 46 BCE as a reform of the Roman calendar. Furthermore, it set the year at 365.25 days by adding a leap day every 4 years without exception. This was more accurate than earlier Roman calendars but still slightly too long — gaining approximately 11 minutes per year over the true solar year.
By 1582, the Julian calendar had drifted 10 days ahead of the solar equinox. Furthermore, this meant the vernal equinox — originally set at 21 March by the Council of Nicaea in 325 CE — was falling on 11 March. Pope Gregory XIII reformed the calendar to correct this drift and prevent it from recurring. Moreover, the Gregorian reform removed the extra days and introduced the century-year exception to the leap year rule.
Old Style and New Style dates
Historians use "Old Style" (OS) for Julian calendar dates and "New Style" (NS) for Gregorian dates. Furthermore, many historical documents from countries that switched late — England, Russia, Eastern Orthodox countries — use OS dates. Converting these to NS for modern calendars requires knowing the exact offset for the year in question. Moreover, some historical documents include both styles, written as "3/14 November 1700" — Julian date first, Gregorian second.
Why Julian to Gregorian conversion matters
Historical research regularly requires date conversion. Documents from England before September 1752, Russia before February 1918 and Greece before 1923 use Julian dates. Furthermore, comparing events from these countries with contemporaneous Gregorian-calendar events requires converting all dates to a common system. Moreover, genealogical records, legal documents and ecclesiastical records from these periods all use Julian dating.
The Eastern Orthodox Church still uses the Julian calendar for its liturgical calendar. Furthermore, Christmas and Easter in the Orthodox tradition fall on Julian calendar dates — which correspond to Gregorian dates 13 days later in the 20th and 21st centuries. This is why Orthodox Christmas falls on 7 January in the Gregorian calendar while Western Christmas is 25 December. Moreover, this difference will increase to 14 days from the year 2100 onwards.
How Newton's birth year illustrates the date ambiguity
Isaac Newton was born on 25 December 1642 in the Julian calendar — the date recorded in English documents of the period. Furthermore, in the Gregorian calendar, this date is 4 January 1643. This is why Newton appears in some sources with a birth year of 1642 and in others with 1643. Moreover, this ambiguity appears throughout English historical records from before September 1752 — the date England adopted the Gregorian calendar.
Frequently asked questions
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