Glossary of Julian Date Terms

Plain-language definitions for every technical term used across these pages, listed alphabetically. Follow any cross-reference to the relevant guide.

Barycentric Julian Date (BJD)
A Julian Date corrected for the light-travel-time difference between the observer and the solar-system barycenter (the center of mass of the Sun and planets). BJD removes the roughly ±500-second variation caused by Earth’s orbital motion and is the standard for high-precision astronomical timing. The preferred time scale is TDB, giving BJD-TDB. See the astronomy guide.
Best-by date
A date through which a manufacturer expects stated quality under specified storage conditions. It is different from an internal lot code and is not safely reconstructed from an undocumented numeric substring.
Born-on date
A consumer-facing production-date marking publicly used by Anheuser-Busch for Budweiser. Packaging and formats can change, so current package or manufacturer guidance controls. See the Budweiser code guide.
Day of Year (DOY)
The ordinal number of a day within its year, from 1 (January 1) to 365 or 366 (December 31, depending on leap year). Also called the ordinal date. Informally — and very commonly — called a “Julian date” in manufacturing and logistics. The DOY is the foundation of the YJJJ, YYDDD, and YYYYDDD formats. See formats.
DLMS (Defense Logistics Management System)
The modern successor to the legacy MILSTRIP system in U.S. military logistics. DLMS transactions favor the seven-digit YYYYDDD format over the four-digit YJJJ, eliminating the ten-year rollover ambiguity. See the military guide.
DODAAC (Department of Defense Activity Address Code)
A six-character code that uniquely identifies a military unit or activity. It forms part of the 14-character MILSTRIP document number, alongside the YJJJ Julian date and a serial number. See the military guide.
Epoch
The reference point from which a time system counts. The epoch of the Julian Day Number is noon UTC on January 1, 4713 BC. The epoch of the Modified Julian Date is midnight UTC on November 17, 1858. The epoch of J2000.0 is noon TT on January 1, 2000. The epoch of GPS time is midnight UTC on January 6, 1980.
FITS (Flexible Image Transport System)
The standard file format for astronomical images and data tables. FITS headers record the time of observation in keywords such as MJD-OBS or DATE-OBS. Virtually every research telescope writes FITS files. See the astronomy guide.
Fliegel–Van Flandern algorithm
A compact formula for computing the Julian Day Number from a Gregorian calendar date, published by Henry F. Fliegel and Thomas C. Van Flandern in 1968. It is a standard reference used by the U.S. Naval Observatory and is the basis of most converter implementations. See the developer snippets.
Freshness window
A quality period defined by a manufacturer for a particular product and storage condition. It varies widely and should not be inferred from a lot code or applied as a universal shelf-life formula. Use the package date and the manufacturer’s storage guidance.
Heliocentric Julian Date (HJD)
A Julian Date corrected for the light-travel time between the observer and the center of the Sun. An older alternative to BJD, still seen in some contexts but largely superseded by BJD-TDB for precision work. See the astronomy guide.
IERS (International Earth Rotation and Reference Systems Service)
The body that publishes Earth Orientation Parameters — including polar motion, UT1−UTC, and length of day — in tables indexed by MJD. See the astronomy guide.
J2000.0
The standard astronomical epoch: Julian Date 2,451,545.0, corresponding to 2000 January 1 at 12:00 TT (Terrestrial Time). Star catalogs such as Hipparcos and Gaia express positions at J2000.0. The “J” prefix denotes Julian years of exactly 365.25 days. See the astronomy guide.
Julian calendar
The solar calendar introduced by the Roman dictator Julius Caesar in 46 BC, with a leap day every four years. It is not the source of the Julian date system; the two simply share the name “Julian.” See the history page.
Julian Date (JD)
The Julian Day Number extended with a fractional part for the time of day. Because the day begins at noon, the fraction is 0.5 at midnight UTC and 0.0 at noon UTC. Used in astronomy for precise time-stamping of observations. See formats.
Julian Day Number (JDN)
An integer giving the continuous count of days since noon UTC on January 1, 4713 BC (in the proleptic Julian calendar). The concept originates with Joseph Scaliger in 1583. Used in astronomy, spacecraft operations, and any context needing a universal, unambiguous day index. See formats.
Julian Period
The 7,980-year cycle Scaliger defined as the product of the 28-year solar cycle, the 19-year Metonic cycle, and the 15-year Roman indiction. The current period began on January 1, 4713 BC and ends in AD 3268. See the history page.
Meeus, Jean
Belgian astronomer and author of Astronomical Algorithms, the widely cited reference that documents the Fliegel–Van Flandern method (Chapter 7) and many other date-computation techniques used in observational astronomy.
Metonic cycle
A 19-year period after which the Moon’s phases recur on approximately the same calendar dates. Named for Meton of Athens (432 BC). One of the three cycles whose product defines the Julian Period. See the history page.
MILSTRIP (Military Standard Requisitioning and Issue Procedures)
The legacy U.S. military supply-requisition system, defined in DoD 4000.25-1-M. Its 14-character document number embeds a four-digit YJJJ Julian date. Being superseded by DLMS. See the military guide.
Modified Julian Date (MJD)
MJD = JD − 2,400,000.5, defined by the Smithsonian Astrophysical Observatory in 1957. It shortens the count to five digits for modern dates and shifts the day boundary from noon to midnight. MJD 0.0 = 1858 November 17 at 00:00 UT. Used by GPS, IERS, VLBI, and spacecraft telemetry. See formats.
Ordinal date
The ISO 8601 term for the day of year, written YYYY-DDD (for example, 2026-169). Equivalent to DOY. See formats.
Production date
The day a product was manufactured. Some documented lot codes encode it as an ordinal date, while others do not expose it in a consumer-readable form. It is distinct from a best-by date. See the how-to-read guide.
Proleptic Julian calendar
The Julian calendar extended backward to dates before it actually existed. It is used to express the Julian Day Number epoch of 4713 BC January 1, a date long predating any real calendar. See the history page.
Scaliger, Joseph Justus (1540–1609)
French classical philologist who introduced the Julian Period and the Julian Day count in 1583. He named the system after his father, Julius Caesar Scaliger — not after the Roman dictator. See the history page.
VLBI (Very Long Baseline Interferometry)
A radio-astronomy technique that combines signals from telescopes on different continents to achieve extremely fine angular resolution. VLBI data is timestamped in MJD. See the astronomy guide.
YJJJ
A four-character ordinal date: one digit for the last digit of the year, followed by three digits for the day of year. It repeats every ten years and should only be applied when the format is documented. See formats.
YYDDD
A five-character ordinal date: two digits for the year, followed by three for the day of year. It repeats every century and should only be applied when that order is documented. See formats.
YYYYDDD
A seven-character date code: a full four-digit year followed by the three-digit day of year. The unambiguous, archival-grade alternative to YJJJ, used in DLMS and export supply chains. See formats.

History →

How one name came to cover two unrelated inventions, from 1583 to GPS.

FAQ →

Quick answers to the most common Julian date questions.