Convert Julian Date to Gregorian Date
Convert Julian Day Number, Julian Calendar date, or ordinal date (YYYY-DDD) to Gregorian. Free, offline, client-side, instant and secure.
"Julian date" means three different things. Pick the one you have: a Julian Day Number (astronomical day count from 4713 BC), a Julian Calendar date (Julius Caesar's calendar, used before 1582), or an ordinal date (YYYY-DDD, common in aerospace). Conversion uses Meeus / Fliegel-Van Flandern - accurate from 4713 BC to 9999 AD.
How to Use Convert Julian Date to Gregorian Date
- Identify which "Julian date" you have. A Julian Day Number is a single non-negative number (often with a
.5for noon UTC). A Julian Calendar date is a Y/M/D triple from before 1582 (or before 1752 in English records). An ordinal date is a year plus a day-of-year (1-366). - Pick the matching mode in the "Julian Date Type" selector. The input fields change to match - JDN shows a number box; Calendar shows year/month/day; Ordinal shows year and day-of-year.
- Type your value(s). Defaults are pre-filled with today's Gregorian date so you can see a working example immediately.
- Press Convert (or Ctrl+Enter). The output panel shows the Gregorian date, the equivalent Julian Calendar date, the JDN, the Gregorian ordinal (YYYY-DDD), the day of the week, and the Julian-Gregorian offset for that year.
- Need the reverse? Click Swap ⇄ or change the "Direction" selector to Gregorian → Julian. The input collapses to a single Y/M/D field; output shows all three Julian representations.
- Copy puts the full report on your clipboard. Download saves it as
julian-gregorian-report.txt.
Frequently Asked Questions
What’s the difference between a Julian Day Number and a Julian Calendar date?
A Julian Day Number (JDN) is a continuous count of days since noon UTC on 1 January 4713 BC (proleptic Julian). Astronomers and software use it because you can subtract two JDNs to get an exact day count, ignoring all calendar rules. A Julian Calendar date is a year-month-day expressed in Julius Caesar’s calendar (every 4 years is a leap year, no exceptions) – used by Europe before the 1582 Gregorian reform and by Russia until 1918.
What’s a “Julian date” in NASA / aerospace contexts?
Aerospace, banking, and computing often use “Julian date” to mean an ordinal date: a year plus a day-of-year number, e.g., 2024-136 (day 136 of 2024 = May 15). It’s a common point of confusion – this tool covers all three meanings in the mode selector.
Why is the Julian-to-Gregorian offset different in different centuries?
The two calendars drift apart by ~3 days every 400 years because Julian counts every century year as a leap year, while Gregorian skips three of every four (1700, 1800, 1900 not leap; 2000 is). The offset was 10 days at the 1582 reform, 11 days from 1700-1799, 12 from 1800-1899, 13 from 1900-2099, 14 from 2100-2199, etc. The result panel shows the offset for your specific year.
Is October 1582 really missing 10 days?
Yes, in Catholic Europe. Julian 4 October 1582 was followed by Gregorian 15 October 1582 – October 5 to 14 simply didn’t exist on the calendar that year. Other countries adopted later (Britain in 1752, Russia in 1918), so the missing-days range depends on jurisdiction. This tool converts mathematically using the proleptic Gregorian calendar – it gives the same JDN regardless.
Why use the proleptic Gregorian calendar for dates before 1582?
“Proleptic” means extending the Gregorian rule backward into history (so 1500 is not a leap year because 1500/400 isn’t an integer). It’s the standard ISO 8601 convention and matches astronomical software. If you want the calendar people actually used in 1500 AD, switch to “Julian Calendar” mode.
How do BC years work?
This tool uses astronomical year numbering: year 0 = 1 BC, year -1 = 2 BC, year -100 = 101 BC. There was no year 0 historically – historians write “1 BC” then “1 AD” – but astronomical numbering makes JDN arithmetic clean.
Will 1700-02-29 work?
In Julian Calendar mode, yes – 1700 was a Julian leap year (every 4 years, no exception). In Gregorian mode the tool will reject it because 1700 is not divisible by 400, so February 1700 had only 28 days in the Gregorian calendar.
What’s J2000 and why is it 2451545.0?
J2000.0 is the standard astronomical epoch for star catalogs and orbital elements. It equals JDN 2451545.0, which is exactly noon UTC on 1 January 2000 in the Gregorian calendar (a Saturday). Try entering 2451545 in JDN mode – you’ll see it confirm.
Is my date data uploaded?
No. All arithmetic runs in your browser using integer math – no network calls, no analytics. Open DevTools → Network and watch zero requests fire after the page loads.
Does it work offline?
Yes. Total bundle is under 22 KB. Once loaded, disconnect and keep converting – useful for archival research where you’re working from local sources without internet access.