Julian Day to Unix Timestamp Converter
Convert a Julian Day (JD) or Modified Julian Day (MJD) to a Unix timestamp, ISO 8601, and a readable date. Julian dates are the standard continuous day count used in astronomy and many scientific datasets.
What is a Julian Day?
The Julian Day (JD) is a continuous count of days (and fractions of a day) since noon Universal Time on January 1, 4713 BC in the proleptic Julian calendar. Because it is a single ever-increasing number with no months or timezones, it is the standard way astronomers compute intervals between events. The Modified Julian Day (MJD) is simply JD − 2400000.5, which shifts the origin to midnight on November 17, 1858.
- JD epoch: noon UT, January 1, 4713 BC (proleptic Julian)
- A JD day starts at noon, not midnight — the .5 fraction marks midnight
- MJD = JD − 2400000.5 (origin 1858-11-17 00:00 UTC)
- JD at the Unix epoch (1970-01-01 00:00 UTC) = 2440587.5
How to convert a Julian Day to a Unix timestamp
Subtract the Julian Day of the Unix epoch (2440587.5) and multiply by the number of seconds in a day.
- Unix seconds = (JD − 2440587.5) × 86400
- Unix seconds = (MJD − 40587) × 86400
- Example: 2460263.42592593 → 1700000000 → 2023-11-14 22:13:20 UTC
- Reverse: JD = Unix seconds / 86400 + 2440587.5
JD vs MJD and precision
Watch the half-day offset: because the Julian Day rolls over at noon, an integer JD corresponds to noon UTC, while a “.5” JD corresponds to midnight. MJD avoids this by rolling over at midnight. This converter accepts a JD value and also reports the equivalent MJD in the results.
- Use MJD for modern date math — it lines up with civil midnight
- Floating-point days lose sub-second precision for very large JD values
- Julian Day is unrelated to the Julian calendar despite the shared name
- What is the Julian Day of the Unix epoch?
- January 1, 1970 00:00:00 UTC corresponds to Julian Day 2440587.5.
- How do I convert a Julian Day to a normal date?
- Subtract 2440587.5 and multiply by 86,400 to get a Unix timestamp, then format that as a date.
- What is the difference between JD and MJD?
- The Modified Julian Day is JD minus 2400000.5. MJD starts at midnight on 1858-11-17, while JD starts at noon on 4713 BC.