Tip: Julian Day begins at noon UTC. This calculator returns both Julian Date (JD) and Julian Day Number (JDN).
What is a Julian Day Number?
The Julian Day Number (JDN) is a continuous count of days used by astronomers, researchers, and software systems to avoid calendar confusion. Instead of writing dates in many regional formats, a date is represented as one integer value. This makes date math easy: subtract two JDN values to get the exact number of days between dates.
Closely related is the Julian Date (JD), which includes fractional days. For example, adding time-of-day gives a decimal value, while JDN is the nearest whole day count.
How to use this julian day number calculator
- Enter year, month, day.
- Enter hour, minute, second in UTC.
- Select a calendar mode:
- Auto: Gregorian on and after 1582-10-15, Julian before that.
- Gregorian: always treat input as Gregorian.
- Julian: always treat input as Julian calendar.
- Click Calculate to see JD, JDN, MJD, and weekday.
Why people use Julian day calculations
1) Astronomy and satellites
Astronomical datasets, ephemerides, and orbital computations commonly use JD or JDN. A single continuous day scale simplifies formulas and reduces conversion errors.
2) Historical and archival research
Historians often compare events recorded under different calendar systems. Converting to JDN gives a neutral baseline for sorting and interval analysis.
3) Software and engineering
Developers use Julian day conventions for scheduling, simulation timelines, and interoperable scientific data formats.
JD vs JDN vs MJD (quick guide)
- JD: Continuous count with fractions of a day.
- JDN: Integer day count derived from JD.
- MJD (Modified Julian Date): JD - 2,400,000.5, common in engineering and observatories.
Important note about noon start
Julian Date traditionally starts at 12:00 UTC (noon), not midnight. That is why many midnight timestamps appear with a fractional part ending in .5.
This is expected behavior and not a bug.
Common mistakes to avoid
- Mixing local time with UTC.
- Using Gregorian values for very old dates without checking calendar assumptions.
- Confusing day-of-year formats with Julian Day Number (they are different concepts).
FAQ
Does this work for old dates?
Yes. Use the calendar selector to control whether you want Julian, Gregorian, or automatic switching near the historical reform.
Can I input fractional seconds?
Yes. Seconds support decimal values for higher precision.
Does year 0 work?
Yes. The underlying math uses astronomical year numbering, where year 0 corresponds to 1 BCE.