Julian Date (JD)
2461254.5
Astronomical day count at 00:00 UTC
Free genealogy date tool
Free Julian Date Converter
A Julian date converter changes a Gregorian calendar date into an astronomical Julian Date, Julian Day Number, or ordinal day-of-year code—and converts those values back. Use it to decode unfamiliar dates in genealogy, archival, scientific, and military records.
Julian Date and JDN5-digit YYDDD7-digit YYYYDDDUTC-safe conversion
Gregorian to Julian
Convert a calendar date
Results use midnight UTC and the proleptic Gregorian calendar so the same input always produces the same value.
Julian Day Number
2461255
Integer assigned to this civil date
Day of year
214
001 through 365, or 366 in leap years
7-digit ordinal
2026214
YYYYDDD format
5-digit ordinal
26214
YYDDD format; century is omitted
Julian to Gregorian
Reverse a Julian value
Gregorian result
2000-01-01
January 1, 2000
Three formats called “Julian date”
Julian Date (JD) is an astronomical count of days since a fixed epoch. Because each Julian day begins at noon, a civil date at 00:00 UTC has a .5 value.
Julian Day Number (JDN) is the integer identifying the Julian day that contains the date. January 1, 2000 at midnight UTC is JD 2451544.5 and JDN 2451545.
Ordinal dates combine a year with its day number. February 29, 2024 is 2024060 in YYYYDDD form or 24060 in YYDDD form.
Calendar note for historical records
This converter applies Gregorian leap-year rules backward and forward for consistent calculations. That is called the proleptic Gregorian calendar. It does not automatically interpret Old Style Julian calendar dates, because the historical switch to the Gregorian calendar occurred at different times in different countries and communities.
For genealogy evidence, preserve the date exactly as written, note the calendar or numeric convention, and store the converted date as a separate interpretation rather than silently replacing the original.
How to use the converter
Step 1
Choose a Gregorian date
Enter a calendar date to see its astronomical Julian Date, Julian Day Number, day of year, and ordinal date formats.
Step 2
Identify the format
Check whether a source uses JD, JDN, YYDDD, or YYYYDDD. These formats are related but not interchangeable.
Step 3
Reverse a numeric value
Select the matching input format, enter the value, and choose a century when a 5-digit YYDDD code is ambiguous.
Step 4
Save the result with its source
Copy the converted values into your research notes and record which date convention the original source used.
Frequently asked questions
What is a Julian date converter?
A Julian date converter translates a Gregorian calendar date into an astronomical Julian Date or an ordinal day-of-year format. It can also convert those numeric values back into a Gregorian date.
What is the difference between Julian Date and Julian Day Number?
Julian Date is a continuous astronomical count that begins at noon, so midnight UTC ends in .5. Julian Day Number is the integer assigned to the Julian day containing that date.
What is a 5-digit Julian date?
A 5-digit Julian date usually means YYDDD: the last two digits of the year followed by the three-digit day of year. For example, 24060 is February 29, 2024 when the selected century is 2000.
Does this convert dates from the historical Julian calendar?
No. This tool uses the proleptic Gregorian calendar for consistent calculations. A historical Julian calendar date is a different system and may require a location-specific Old Style to New Style conversion.
Why are Julian dates useful in genealogy?
Julian and ordinal date formats appear in scientific records, military files, manufacturing codes, databases, and archival notes. Converting them can help a researcher place an unfamiliar numeric date on a family timeline.
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