Julian Date Calculator
Convert a Gregorian calendar date and UTC time to astronomical Julian Date, Julian Day Number, Modified Julian Date and ordinal day. You can also convert a Julian Date or year-plus-day-of-year back to a calendar timestamp.
Two meanings are kept separate: astronomers use a continuous Julian Date that starts at noon UTC, while logistics and manufacturing often call the ordinal format YYYY‑DDD a “Julian date.” Neither is a conversion to the historical Julian calendar.
Julian Date formulas and definitions
- Julian Date (JD) is a continuous count of days and fractions of a day. Its integer changes at 12:00 UTC.
- Julian Day Number (JDN) here is floor(JD + 0.5), the integer label associated with the civil date.
- Modified Julian Date (MJD) is JD − 2,400,000.5, so its day begins at midnight UTC.
- Ordinal date is the Gregorian year followed by the day number 001–365, or 001–366 in a leap year.
The implementation uses the exact Unix-epoch relationship JD = Unix milliseconds ÷ 86,400,000 + 2,440,587.5. Dates are treated as proleptic Gregorian dates from 1583 through 9999.
Six checked conversions
| Input | Julian / ordinal result | Calendar result |
|---|---|---|
| 2000-01-01 12:00:00 UTC | JD 2451545.000000; MJD 51544.500000 | 2000-001 |
| 1970-01-01 00:00:00 UTC | JD 2440587.500000; MJD 40587.000000 | 1970-001 |
| 2024-02-29 00:00:00 UTC | JD 2460369.500000 | 2024-060 |
| 2023-12-31 23:59:59 UTC | JD 2460310.499988 | 2023-365 |
| Ordinal 2024-060 | Leap-year day 60 | 2024-02-29 00:00:00 UTC |
| JD 2451544.500000 | JDN 2451545 | 2000-01-01 00:00:00 UTC |
Precision and limits
- All displayed times are UTC. The calculator does not infer a local time zone or daylight-saving offset.
- Julian Date is displayed to six decimals, about 0.0864 seconds per final decimal. Reverse conversion is rounded to the nearest second for display.
- The range starts at 1583 to avoid mixing Gregorian-reform conventions. Historical sources can use different calendars for earlier dates.
- YYDDD discards the century. Use YYYYDDD when a code must be unambiguous.
- Astronomical time standards such as TT, TAI and leap-second handling are outside this civil UTC calculator.
Reference: U.S. Naval Observatory Julian Date converter and definition.
Related calculators: Date Calculator, Age Calculator and Hours Calculator.
Questions
Is YYYYDDD an astronomical Julian Date?
No. It is an ordinal Gregorian date code. This calculator shows both formats but labels them separately.
Why does JD end in .5 at midnight?
The astronomical Julian day begins at noon UTC. Midnight is therefore halfway through the numbered Julian day.
What is the difference between JD and JDN?
JD can include a fractional day and represents an instant. JDN is an integer day label obtained here with floor(JD + 0.5).
Does this convert to the Julian calendar?
No. Inputs and outputs are Gregorian calendar dates. “Julian” in Julian Date refers to the continuous astronomical day count.