Julian Date Converter — JDN, Julian Calendar & Ordinal Date

What is a Julian date?

'Julian date' means three different things, which is why so many converters are confusing. (1) Julian Day Number (JDN): a continuous count of days used in astronomy, starting at noon on January 1, 4713 BC. (2) A date in the Julian calendar: the 'Old Style' calendar used before the Gregorian reform (still used by some Orthodox churches). (3) An ordinal date: the year plus the day-of-year, like 26279 (yyddd) — used on food packaging, in the military, and in ERP systems like JD Edwards. This tool handles all three and labels each clearly.

What is the Julian Day Number (JDN)?

The Julian Day Number is the number of days elapsed since noon Universal Time on Monday, January 1, 4713 BC (Julian proleptic calendar). Astronomers use it because it makes date arithmetic trivial — the difference between two JDNs is the exact number of days between them, with no month or leap-year complications. For example, J2000.0 (noon, January 1, 2000) is JDN 2451545.

What is the difference between Julian Day Number and the Julian calendar?

They share a name but are unrelated concepts. The Julian Day Number is a day-count system for astronomy (a single integer per day, e.g. 2461320). The Julian calendar is the calendar Julius Caesar introduced in 45 BC, with a leap year every 4 years and no exceptions — it drifts about 3 days every 400 years versus the seasons, which is why the Gregorian calendar replaced it in 1582. Mixing them up is the most common error in 'Julian date' converters.

What is the Julian date today?

Use the 'Today' button in any tab above for the live value. As a reference anchor: January 1, 2000 at noon UTC (J2000.0) is Julian Day 2451545.0, which is Modified Julian Date 51544.5. In ordinal form, October 6, 2026 is day 279 of 2026, written 2026279 (yyyyddd), 26279 (yyddd), or 126279 in JD Edwards Cyyddd format.

How do I convert a date to Julian Day Number?

Use the Julian Day Number tab above, or the Fliegel–Van Flandern algorithm: with integer division truncating toward zero, JDN = (1461×(y+4800+(m−14)/12))/4 + (367×(m−2−12×((m−14)/12)))/12 − (3×((y+4900+(m−14)/12)/100))/4 + d − 32075. For January 1, 2000 this gives 2451545. The tool above implements this exactly and round-trips without drift.

What is Modified Julian Date (MJD)?

Modified Julian Date is MJD = JD − 2400000.5, introduced by the Smithsonian Astrophysical Observatory in 1957 to make the numbers smaller and start the day at midnight instead of noon. J2000.0 (JD 2451545.0) is MJD 51544.5. Satellite tracking, GPS-adjacent systems, and many scientific datasets use MJD.

What is an ordinal date (yyddd)?

An ordinal date writes the year followed by the day-of-year: 3 digits for the day (001–366). October 6, 2026 is day 279, so yyyyddd = 2026279 and yyddd = 26279. Manufacturers print these on food and pharmaceuticals as compact date codes, the military and logistics use them for unambiguous day-stamping, and mainframe/ERP systems store dates this way.

What is the JD Edwards Julian date format?

JD Edwards (JDE) EnterpriseOne stores dates as Cyyddd: a century flag (0 = 1900s, 1 = 2000s), two-digit year, and three-digit day-of-year. October 6, 2026 becomes 126279 (1 = 2000s, 26 = year, 279 = day). The Ordinal Date tab above converts to and from this format, which is what 'julian date converter for jde' searches are asking for.

How do food manufacturers use Julian dates?

Food, beverage, egg, and pharmaceutical producers stamp packages with ordinal 'Julian' codes like 26279 (yyddd) or 2026279 (yyyyddd) to mark production or best-by dates in a compact, machine-friendly form. Beer, cigarettes (MRE rations use similar coding), and egg cartons commonly carry these. Enter the code in the Ordinal Date tab's reverse converter to read the calendar date.

How does the military use Julian dates?

US military logistics and the Navy/Marines/Army use ordinal Julian dates (yyddd) for supply, maintenance, and message dating because they sort chronologically as plain numbers and avoid month-name ambiguity. A date-time group (DTG) like 061200Z OCT 26 pairs the ordinal-style day (06) with Zulu (UTC) time. This is the intent behind 'julian date converter us navy / army / usmc / military' searches.

What is the Excel formula for Julian date?

For the Julian Day Number from an Excel date in A1: =INT(A1)+2415019 (Excel's serial date plus the offset; verified: 2000-01-01 serial 36526 gives 2451545). For the ordinal yyyyddd form: =YEAR(A1)*1000+(A1-DATE(YEAR(A1),1,1)+1). For yyddd: =TEXT(A1,"yy")&TEXT(A1-DATE(YEAR(A1),1,1)+1,"000"). Reverse (ordinal in A1, as text): =DATE(LEFT(A1,4),1,1)+RIGHT(A1,3)-1.

What is heliocentric Julian date (HJD)?

Heliocentric Julian Date corrects the observed Julian Date to the Sun's center, removing up to ±8.3 minutes of light-travel time as Earth orbits the Sun. Variable-star observers (e.g. AAVSO) use HJD so observations months apart are comparable. Enter the observation date plus the target's RA/Dec in the Heliocentric JD tab; the correction uses a low-precision solar ephemeris (about ±30 s accuracy).

What is barycentric Julian date (BJD)?

Barycentric Julian Date corrects to the Solar System barycenter (its center of mass) instead of the Sun's center — the modern standard for exoplanet transit timing, replacing HJD. BJD differs from HJD by up to about ±4 seconds due to the Sun's wobble around the barycenter (mostly Jupiter's pull). Computing publication-quality BJD needs a JPL ephemeris, which is beyond an offline web tool's honest scope — the Heliocentric tab discloses its ±30 s accuracy for this reason.

Do Julian dates handle leap years?

The Julian Day Number needs no leap-year logic at all — it is just a day count. Ordinal dates do: in leap years the day-of-year runs to 366 (e.g. December 31, 2024 = 2024366). The Julian calendar's leap rule is simpler than the Gregorian one — every 4th year, including century years like 1900 and 2100 — which is exactly why it drifted from the seasons and was replaced.

What is J2000.0?

J2000.0 is the standard astronomical epoch: noon Terrestrial Time on January 1, 2000, defined as Julian Date 2451545.0 exactly. Star catalogs, orbital elements, and coordinate systems are referenced to J2000.0. NASA, JPL, and the US Naval Observatory all use Julian Dates anchored to this epoch.

Is my data sent to a server?

No. Every conversion runs locally in your browser with JavaScript — the Fliegel–Van Flandern calendar math, ordinal parsing, and heliocentric correction all execute on your device. Nothing is uploaded, which is why the tool works offline once loaded.

How do I find the difference between two Julian dates?

Subtract them — the difference of two Julian Dates is the exact elapsed time in days, with no month-length or leap-year arithmetic. For example, JD 2461320 minus JD 2451545 is 9775 days. Use the 'Difference between two Julian Dates' section in the Julian Day Number tab above; it also shows hours and seconds.

Can I convert JD Edwards Julian dates in SQL?

Yes. A JDE Cyyddd value like 126279 decodes as: full year = century-flag×100 + 1900 + yy (so 1×100+1900+26 = 2026), then add (ddd−1) days to January 1st. In T-SQL: DATEADD(day, CAST(RIGHT(jde_date,3) AS INT)-1, DATEFROMPARTS(CAST(LEFT(jde_date,1) AS INT)*100+1900+CAST(SUBSTRING(jde_date,2,2) AS INT),1,1)). The JD Edwards section above explains the format step by step.