The Seven Julian Date Formats
"Julian date" means at least seven different things. Here is each format, who uses it, and its value right now — Thursday, September 17, 2026. Confusing two of these is the single most common Julian date mistake.
| Format | Today |
|---|---|
| DOY Day of Year (Ordinal Date) | 260 |
| YJJJ 4-digit (YJJJ) | 6260 |
| YYDDD 5-digit (YYDDD) | 26260 |
| YYYYDDD 7-digit (YYYYDDD) | 2026260 |
| JDN Julian Day Number (JDN) | 2461301 |
| JD Julian Date (JD) | 2461300.82790 |
| MJD Modified Julian Date (MJD) | 61300.32790 |
DOY — Day of Year (Ordinal Date)
Today: 260The simplest form: the count of days since January 1, which is always day 1. December 31 is day 365 (or 366 in a leap year). Often loosely called the "Julian day" even though it carries no year information at all.
Used by: Spreadsheets, aviation logs, general business
YJJJ — 4-digit (YJJJ)
Today: 6260A single year digit followed by the three-digit day of year. It is compact but ambiguous across decades — a "6" could mean 2016 or 2026. Never assume an undocumented package code uses this layout.
Used by: Some packaging and legacy logistics systems
YYDDD — 5-digit (YYDDD)
Today: 26260Two year digits plus the three-digit day of year. It appears in some business and production systems and is unambiguous only when the correct century is known.
Used by: Some databases, business systems, and lot codes
YYYYDDD — 7-digit (YYYYDDD)
Today: 2026260A full four-digit year plus the day of year. It avoids the decade and century ambiguity of shorter ordinal formats and is useful in long-lived records.
Used by: Data exchange, logistics, and archival records
JDN — Julian Day Number (JDN)
Today: 2461301A continuous integer count of days since 4713 BC January 1 at noon UTC (proleptic Julian calendar). Subtract one JDN from another to get the exact number of days between any two dates — the reason astronomers adopted it.
Used by: Astronomy, FITS files, date arithmetic
JD — Julian Date (JD)
Today: 2461300.82790The JDN extended with a fractional day for the time of day. The epoch is noon, not midnight, so a whole night of observation falls inside a single integer day. At 00:00 UTC the JD ends in .5.
Used by: Observatories, spacecraft, scientific timing
MJD — Modified Julian Date (MJD)
Today: 61300.32790JD minus 2,400,000.5. The subtraction shifts the day boundary to civil midnight and shrinks the number to five digits, which is why GPS and Earth-orientation systems use it.
Used by: GPS, IERS, satellite telemetry
The two families, and why they collide
All seven of these formats fall into two unrelated families that happen to share a name. The ordinal day-of-year codes (DOY, YJJJ, YYDDD, YYYYDDD) describe a calendar day with no time-of-day component; they were adopted by industry for compact, sortable date stamps. The astronomical counts (JDN, JD, MJD) describe a continuous timeline of days and fractions since 4713 BC; they were designed for computing intervals between events.
The collision happens because both got called "Julian." The astronomical system is named after Joseph Scaliger's father, Julius Caesar Scaliger — not the Julian calendar. The ordinal codes picked up the name much later, from the U.S. military supply chain in the punched-card era, and from there spread into the food industry. When someone says "today's Julian date," the right answer depends entirely on whether they are stamping a soup can or scheduling a telescope.
Rule of thumb
If the number has 3–7 digits and no decimal point, it is an ordinal day-of-year code. If it has a decimal point and is in the millions (JD) or tens of thousands (MJD), it is astronomical.
Convert any date →
Get all seven formats for any calendar date, or reverse a code back to a date.
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Read the Julian date stamped on packaging from 15 major brands.