A Julian Day value counts days from noon on 4713 BCE-11-24 in the proleptic Julian calendar, with fractional days for time. Its source epoch and unit need to travel with it; the number alone cannot tell you which clock it belongs to.
Conversion rule
Unix milliseconds = (JD − 2440587.5) × 86400000
The converter uses that relationship to return Unix seconds, epoch milliseconds, ISO 8601 UTC, and a readable UTC value. When investigating a log or export, keep the original value so you can check the conversion later.
Where mistakes happen
Julian Day begins at noon, not midnight. The .5 in the Unix-epoch constant is the detail that prevents a 12-hour error.
A safer workflow
- Identify the platform and field name before doing arithmetic.
- Preserve the original value as a string when it may exceed JavaScript safe-integer precision.
- Convert once at the boundary, then store or compare a declared Unix unit.
- Format in a named timezone only when presenting the resulting instant to a person.
Related format converters
Frequent questions:
- Q: What is the Julian Day of the Unix epoch?
- A: January 1, 1970 00:00:00 UTC corresponds to Julian Day 2440587.5.
- Q: How do I convert a Julian Day to a normal date?
- A: Subtract 2440587.5 and multiply by 86,400 to get a Unix timestamp, then format that as a date.
- Q: What is the difference between JD and MJD?
- A: 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.