Julian Date Today
Today's Julian Day Number — the continuous day count used by astronomers, aviation, and mainframe systems. Also shows the common YYDDD ordinal format.
Gregorian → Julian Day
Enter any Gregorian date to find its Julian Day Number.
What is the Julian Day Number?
The Julian Day Number (JDN) is a continuous count of days since 1 January 4713 BCE (in the Julian proleptic calendar). It's the standard time-keeping system used by astronomers, and — in a different form — by aviation, military, and mainframe systems.
The beauty of the Julian Day Number is that it ignores months, years, and leap rules entirely. To find the number of days between any two events in history, you simply subtract one JDN from the other. No quirks, no edge cases, no century leap-year rules.
Two things called "Julian date"
Confusingly, there are two unrelated things both called "Julian date":
- The astronomical Julian Day Number. A giant number like 2461154, counting continuous days since 4713 BCE. Used in astronomy and scientific computing.
- The mainframe / aviation "Julian date." A five-digit number like 26113, where the first two digits are the year and the last three are the day of the year (1–366). Used in manufacturing date codes, IBM mainframes, and aviation flight plans.
This page shows both.
Where did the astronomical Julian Day come from?
It was invented by Joseph Scaliger in 1583 and named after his father, Julius Caesar Scaliger (not Julius Caesar the Roman). Scaliger chose 1 January 4713 BCE because it was the last time three important astronomical cycles coincided — the 28-year solar cycle, the 19-year Metonic lunar cycle, and the 15-year Roman indiction cycle.
Where you'll see Julian dates today
- Astronomy — every observation is timestamped with a Julian Date (often with the fractional part for the time of day).
- Food manufacturing — expiration date codes often use YYDDD format stamped into the package.
- Aviation — flight plans and maintenance logs use Julian dates for unambiguous day identification.
- Mainframes — legacy IBM systems widely use YYDDD format for business dates.
The YYDDD "Julian date" format in practice
When you buy food in a supermarket and see a mysterious code like 26113 stamped on the package, that's an "industrial Julian date." The first two digits are the year (2026); the last three are the day of the year (113, meaning April 23). This compact format is used by:
- Food manufacturers — production dates and expiration codes
- Aviation — maintenance logs, inspection dates
- Mainframes — IBM z/OS and legacy systems store dates as YYDDD in countless business applications
- Military — equipment stamps, ammunition lot codes
- Pharmaceutical — batch numbers, expiration tracking
Astronomical Julian Dates and fractional days
Astronomers add a fractional component to the Julian Day Number to pinpoint a specific moment in time. The convention is that the Julian Day starts at noon UTC, not midnight. So:
- JD 2461154.0 = noon UTC on 23 April 2026
- JD 2461154.5 = midnight UTC ending 23 April / starting 24 April
- JD 2461154.25 = 18:00 UTC on 23 April
Why noon? Because astronomers traditionally observe at night — keeping a whole night of observations within a single JD date makes record-keeping easier. If the day rolled at midnight, your observation log would split across two dates every night.
Modified Julian Date (MJD)
Because JDN values are inconveniently large (we're currently past 2,461,000), astronomers often use the Modified Julian Date: MJD = JDN − 2400000.5. This starts from 17 November 1858 at midnight UTC and is preferred for modern observations. Today's MJD is about 60794 — much easier to work with.
History: who was Scaliger?
The Julian Day system was invented by Joseph Justus Scaliger, a 16th-century French humanist and chronologist, and published in 1583. He named it after his father, Julius Caesar Scaliger, not after Julius Caesar the Roman. The confusion persists to this day.
Scaliger chose the year 4713 BCE as the starting point because it was the last time three historically important astronomical and calendrical cycles all began on the same day: the 28-year solar cycle, the 19-year Metonic lunar cycle, and the 15-year Roman indiction cycle. Their product, 28 × 19 × 15 = 7,980 years, is the length of the "Julian Period" — and 4713 BCE + 7,980 = 3,268 CE, when the Julian Period will next repeat.