Convert Gregorian Date to Julian Date

Both calendars count the same physical days, but they assign different labels. The Julian calendar (Julius Caesar, 45 BC) inserts a leap day every 4 years, which overcounts solar years by ~11 minutes each. By 1582 this had drifted 10 days, so Pope Gregory XIII introduced the Gregorian calendar with a modified leap rule (skip century years unless divisible by 400). Today, a Gregorian date is 13 days ahead of the same physical day’s Julian date.

It’s 13 because 13 full “skips” of the century-year leap have accumulated since the calendars diverged: 1700, 1800, 1900 (skipped in Gregorian but leap in Julian, so +3), plus the original 10-day jump in 1582. The offset will become 14 days on March 1, 2100 (Julian treats 2100 as a leap year; Gregorian does not). Then 15 days on March 1, 2200. And so on – a new skip every non-divisible-by-400 century year.

A continuous count of whole days since November 24, 4714 BC (proleptic Gregorian). It’s the standard astronomy reference because it avoids calendar complications entirely – just a plain integer. Today (Gregorian 2026-04-24) is JDN 2,461,155. JDN is different from “Julian calendar date” (which shares a name but isn’t a number).

It means applying the Gregorian calendar’s rules backward in time, before the calendar was actually adopted in 1582. Historically, dates before that would have been recorded on the Julian calendar. Proleptic Gregorian is a mathematical convenience – it gives a consistent reference system for computation. Historians usually state which convention they’re using.

Because those 10 Gregorian dates never historically occurred. When Pope Gregory XIII’s reform took effect, the world jumped from Thursday October 4 (Julian) directly to Friday October 15 (Gregorian), dropping the lost 10 days to realign with the solar year. If you enter any date in that range, the tool still computes a result using proleptic math, but warns you that no historical document ever bore such a date.

Several Eastern Orthodox churches – including the Russian, Serbian, Georgian, and Jerusalem Patriarchates – use the Julian calendar for liturgical purposes. That’s why Orthodox Christmas falls on January 7 in the Gregorian calendar (December 25 Julian + 13-day offset). The Berber calendar of North Africa also retains Julian dating.

No. Each physical day has one weekday, regardless of which calendar labels it. Gregorian 2026-04-24 and Julian 2026-04-11 are the same Friday. The conversion shifts the date label; it doesn’t move the day.

Because the offset changes at specific calendar events – March 1 of non-leap-400 century years (1700, 1800, 1900, 2100, 2200, 2300). A naive shortcut like offset = century − century/4 − 2 fails in the first two months of those years. This tool uses the Richards “Mapping Time” Julian Day Number formulas, which handle every edge case correctly by going through JDN as an intermediate step.

No. All arithmetic runs in your browser’s JavaScript engine. No input leaves your device, no cookies are set, no analytics are transmitted. You can open your browser’s Network tab to confirm: no requests fire during a conversion. The tool works offline after the initial page load.

Yes, if you type the ISO format with a negative year in the free-text mode. Astronomical-year numbering is used: year 1 = AD 1, year 0 = 1 BC, year −1 = 2 BC, and so on. The date picker itself may not accept negative years in all browsers, so you may need to type directly. Julian and Gregorian diverge less the further back you go; before about 200 AD the offset is zero.