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What Is a Julian Date?
The Julian Day Number (JDN) is a continuous count of days since the beginning of the Julian Period — noon on January 1, 4713 BCE (Julian calendar), which corresponds to JDN 0. It was introduced by scholar Joseph Justus Scaliger in 1583 and is named after his father Julius Caesar Scaliger — not Julius Caesar or the Julian Calendar.
Julian Dates are used extensively in astronomy, satellite tracking, military operations, computing, and scientific research because they eliminate the ambiguity of month/day/year systems and allow direct arithmetic on dates — subtracting two Julian Day Numbers gives the exact number of days between them.
The Julian Date (JD) extends the integer JDN to include a decimal fraction representing the time of day, where 0.5 = midnight and 0.0 = noon. JD 2451545.0 corresponds to noon on January 1, 2000 — the J2000.0 epoch used as the reference point in modern astronomy.
Julian Date Variants Explained
The full Julian Date including decimal fraction for time. JD 0.0 = noon on 1 Jan 4713 BCE. The fundamental reference system.
The integer part of JD. Used when only the date (not time) matters. Advances at noon UTC each day.
MJD = JD − 2,400,000.5. Defined by the Smithsonian Astrophysical Observatory in 1957. Starts at midnight (not noon), uses smaller numbers, fits in 5 digits for most modern dates.
Days elapsed since noon on 1 January 2000 (JD 2,451,545.0). Used in satellite and spacecraft navigation calculations.
RJD = JD − 2,400,000. Similar to MJD but starts at noon. Used in some astronomical tables.
Seconds since 1 Jan 1970 00:00:00 UTC. Used in computing. Related to JD: UNIX = (JD − 2,440,587.5) × 86,400.
Conversion Formula
The standard algorithm to convert a Gregorian date to Julian Day Number (valid for dates after 15 Oct 1582):
The Julian calendar uses a simpler formula (no century correction) valid for all dates before the Gregorian reform.
How to Use This Converter
- 1Choose your conversion direction — Gregorian → Julian Day, Julian Day → Gregorian, or Julian Calendar Date → Julian Day.
- 2Enter your date — use the date picker or type directly. For Julian Day Number mode, enter the JDN as an integer or decimal (e.g. 2451545.5 for midnight).
- 3Results appear instantly — see JDN, full Julian Date, Modified Julian Date, J2000, Reduced JD, UNIX timestamp, day of week, and more.
- 4Use quick examples — famous dates like the Moon landing (JDN 2,440,423) and the J2000 epoch are pre-loaded for reference.
Worked Example
Convert 20 July 1969 (Apollo 11 Moon landing) to Julian Day Number:
Who Uses Julian Dates?
Planet positions, eclipse predictions, and star catalogs all use JD for date arithmetic across centuries.
TLE (Two-Line Element) orbital data uses epoch in Julian Date format for precise orbit calculations.
US military uses Julian Date for food expiration dates and supply chain tracking (YYDDD format).
Database systems and programming languages use JD-based arithmetic for date difference calculations.
Industrial date coding on food, pharmaceutical, and aerospace products often uses Julian Date (DDD of year).
Converting between Gregorian and Julian calendar dates for historical events before 1582.
Frequently Asked Questions
These are two completely different things that share the name “Julian.” The Julian Calendar is the predecessor to the Gregorian Calendar — introduced by Julius Caesar in 46 BCE, it has a leap year every 4 years with no century correction, making it 11 minutes longer than the solar year. The Julian Date (JD) is a continuous day count used in astronomy, named after Joseph Scaliger’s father Julius Caesar Scaliger — it has nothing to do with the Julian Calendar or Julius Caesar.
When the Julian Day system was established for astronomical use in the 19th century, astronomers worked at night. Starting the day at noon meant that an entire night’s observations all fell within the same Julian Day — making record-keeping cleaner. A day starting at midnight would split each night’s work across two different dates. While inconvenient for other uses, this noon convention remains the standard definition. The Modified Julian Date (MJD) was created partly to address this — it starts at midnight.
J2000.0 is the standard astronomical epoch corresponding to noon on 1 January 2000 UTC (JD 2,451,545.0). It is the reference point for modern star catalogs, planetary ephemerides, and spacecraft navigation. When astronomers give a star’s position, they typically specify it relative to the J2000.0 coordinate system. Satellite TLE data expresses orbital parameters as changes from a reference epoch, often measured in days from J2000. The J2000 field in this calculator shows how many days before or after noon on 1 Jan 2000 your date falls.
Food packaging Julian Dates are not the same as astronomical Julian Day Numbers. They typically use a simplified format: YYDDD or YYYYDDD where DDD is the day of the year (1–365 or 1–366). For example, 25032 means the 32nd day of 2025 — February 1, 2025. To convert: take the year, find January 1 of that year, then count forward DDD−1 days. This calculator does not use the food industry format — it uses the full astronomical Julian Day Number which is a continuous count from 4713 BCE.
The Gregorian calendar reform was announced by Pope Gregory XIII and took effect on 15 October 1582. The day before — 4 October 1582 (Julian Calendar) — was immediately followed by 15 October 1582 (Gregorian Calendar), skipping 10 days to realign the calendar with the solar year. This means dates from 5 October to 14 October 1582 do not exist in the Gregorian calendar. Different countries adopted the reform at different times — Britain and its colonies (including America) did not switch until 1752, which is why George Washington’s birthday changed from February 11 to February 22.
The UNIX timestamp counts seconds since 1 January 1970 00:00:00 UTC (the “UNIX epoch”). Its relationship to Julian Date is: UNIX = (JD − 2,440,587.5) × 86,400. The UNIX epoch corresponds to JD 2,440,587.5. UNIX timestamps are widely used in computing — virtually every programming language and database can work with them. They are precise to the second, while Julian Dates are typically quoted to 5 decimal places (precision of about 1 second). The “Year 2038 problem” occurs because 32-bit UNIX timestamps overflow on 19 January 2038 — equivalent to JD 2,464,731.0.