History of the Julian Date

The problem Scaliger was trying to solve

In the late 16th century, European scholars were engaged in a vast project of comparative chronology. Egyptian, Babylonian, Greek, Roman, Hebrew, and Persian histories each used a different calendar, with different year lengths, month names, and rules for inserting leap days. Converting a date from one calendar to another was an error-prone hand calculation, and a single mistake could misplace a historical event by months or years.

Joseph Justus Scaliger (1540–1609), a French classical philologist widely regarded as a founder of modern chronology, set out to solve this in his monumental work De emendatione temporum (“On the Correction of Chronology”), published in 1583. His solution was characteristically simple: instead of converting between calendars, assign every day in history a unique sequential number. Once you know the number for any date in any calendar, you can compare it to the number for any date in any other calendar by simple subtraction.

The Julian Period: 7,980 years of unambiguous days

Scaliger needed a starting point for his day count — an epoch falling before the earliest event in any recorded chronology. He combined three cycles that scholars had long used to organise time:

• The 28-year solar cycle — the period after which the days of the week repeat on the same dates in the Julian calendar.
• The 19-year Metonic cycle — the period after which the Moon’s phases fall on approximately the same calendar dates, named for the Athenian astronomer Meton (432 BC) and the basis of lunisolar calendars.
• The 15-year Roman indiction — a fiscal cycle used for taxation in the Roman Empire and later for dating church documents.

Multiplying the three gives 28 × 19 × 15 = 7,980 years. Scaliger computed that the most recent date on which all three cycles simultaneously began a new round was January 1, 4713 BC (in the proleptic Julian calendar). He defined that day at noon as day zero of his “Julian Period” and counted forward. The current period runs from 4713 BC to AD 3268.

Adoption by astronomy

Scaliger’s day count was originally a tool for historians, but astronomers quickly recognised its value. To compute the position of a planet, you need to know exactly how many days separate two observations. With a continuous day number, that calculation is a single subtraction — regardless of whether the observations fell in different months, years, centuries, or even calendar systems.

The astronomer John Herschel helped formalise the use of Julian Day Numbers in his Outlines of Astronomy (1849), and the convention of starting each day at noon rather than midnight took hold, so that a full night of observation falls within a single Julian day. That noon epoch is still used today, and it is the source of most off-by-half-a-day errors when converting between Julian dates and civil time.

By the early 20th century, major observatories used the Julian Day Number as their primary time index. National ephemerides, including those published by the U.S. Naval Observatory, listed Julian Day Numbers alongside calendar dates for astronomical events.

The fractional Julian Date and the Modified Julian Date

As observational astronomy grew more precise, the integer Julian Day Number was no longer enough — astronomers needed the exact time of an observation, not just the day. The Julian Date (JD) extends the day number with a fractional part representing how much of the 24-hour day has elapsed since the previous noon. Because the day starts at noon, midnight falls at fraction 0.5, 6:00 AM at 0.75, and the next noon resets to a whole number.

By the mid-20th century the Julian Date had grown inconveniently large. In 1957 — the year of Sputnik — the JD was around 2,436,000, a seven-digit value that was awkward to write, hard to compare at a glance, and wasteful of space in the fixed-width records used by early computers.

The Smithsonian Astrophysical Observatory defined the Modified Julian Date as MJD = JD − 2,400,000.5. The subtraction does two things at once: it shortens the count to five digits for any modern date, and the −0.5 shifts the day boundary from noon to civil midnight. MJD 0.0 corresponds to 1858 November 17 at 00:00 UT. The MJD was adopted rapidly by the satellite-tracking community, then by GPS, then by the International Earth Rotation and Reference Systems Service (IERS), and today it is the standard time index for essentially all space operations and precision timing.

A separate invention: the punched-card era and the military

While astronomers refined Scaliger’s count, the U.S. military was building its logistics systems around punched-card technology. The MILSTRIP system (Military Standard Requisitioning and Issue Procedures), developed in the late 1950s and formalised in DoD 4000.25-1-M, defined a 14-character document number for every supply requisition — with only a handful of card columns left for the date.

A few columns could not hold a full month-day-year date. The solution was to encode the date as a single year digit followed by the three-digit day of year — the format now known as YJJJ. This was not derived from Scaliger’s Julian Day Number; it was an independent invention driven by the same underlying insight, that a sequential day count is more compact and less ambiguous than a month-day format.

The military called this format a “Julian date” because the day-of-year number had informally been the “Julian day” in logistics circles for years. The name stuck, and from the military it spread to defense contractors, then to the broader manufacturing sector, and eventually to the food industry.

Food and consumer-goods adoption

Manufacturers adopted the ordinal Julian date for the same reasons the military did: compactness and traceability. A four-character YJJJ code fits on a can lid, sorts as an integer in a database, and uniquely identifies the production day within a ten-year window. Combined with plant and shift codes, it gives quality engineers what they need to trace a suspect lot through the supply chain.

Today, ordinal date codes appear on a large share of packaged food, beverages, and consumer products. A common point of confusion is that this code is the production date, not a best-by date — the best-by date is found by adding the product’s freshness window to the production day. The batch decoder reads the codes used by many of the largest brands, and the how-to-read guide walks through it step by step.

GPS, VLBI, and the modern era

The Global Positioning System, declared operational in the 1990s after launches began in 1978, uses a time reference closely tied to the Modified Julian Date. GPS time counts seconds from January 6, 1980 at 00:00 UTC (MJD 44244) and deliberately excludes leap seconds. Each satellite broadcasts its time as a GPS week number plus seconds-of-week, which converts to MJD with simple arithmetic.

Very Long Baseline Interferometry (VLBI) networks — which correlate radio signals from telescopes on different continents — timestamp their data in MJD. The IERS Earth Orientation Parameters, which describe exactly how the Earth’s rotation axis is oriented at any moment, are published as tables indexed by MJD.

In short, the day-numbering idea Scaliger introduced in 1583 to reconcile ancient chronologies is now embedded in the infrastructure of modern life — from the satellites that guide navigation, to the batch codes that support food traceability, to the astronomical databases that catalogue the universe.

Timeline at a glance

1. 46 BC Julius Caesar introduces the Julian calendar — unrelated to the later Julian date system except by the shared name.
2. 1583 Joseph Justus Scaliger publishes De emendatione temporum, introducing the Julian Period with its epoch at 4713 BC January 1.
3. 1849 John Herschel popularises Julian Day Numbers in Outlines of Astronomy and the noon-epoch convention.
4. 1858 November 17 — the date later chosen as the epoch (MJD 0.0) of the Modified Julian Date.
5. 1957 The Smithsonian Astrophysical Observatory defines the Modified Julian Date (MJD = JD − 2,400,000.5).
6. Late 1950s The U.S. military's MILSTRIP system adopts the four-digit YJJJ ordinal date for supply requisitions.
7. 1960s Food and consumer-goods manufacturers begin printing ordinal "Julian date" production codes.
8. 1968 Fliegel and Van Flandern publish their compact integer algorithm for computing the Julian Day Number.
9. 1980 GPS time begins counting from January 6 at 00:00 UTC (MJD 44244), without leap seconds.
10. 1996 Anheuser-Busch introduces the Budweiser "born-on date" in YYDDD format.
11. 2000s The Defense Logistics Management System (DLMS) moves toward the seven-digit YYYYDDD format.
12. Today Julian dates are used daily across manufacturing, logistics, astronomy, and satellite navigation.