Julian Day Converter
Convert a Julian Day Number to a calendar date and Unix timestamp, and back.
Julian Day ⇄ Timestamp
Runs entirely in your browserType in either field — the other updates live.
What is a Julian Day Number?
The Julian Day Number (JDN) counts the number of days since noon UTC on January 1, 4713 BC (proleptic Julian calendar) — a continuous day count used in astronomy, geodesy, and some scientific computing to avoid calendar irregularities.
The Unix epoch (1970-01-01T00:00:00 UTC) corresponds to Julian Day 2,440,587.5. The .5 is there because Julian Days traditionally start at noon, not midnight.
- Epoch: noon UTC, January 1, 4713 BC (proleptic Julian calendar)
- Days begin at noon, not midnight — hence the .5 offset
- Widely used in astronomy and satellite/orbital calculations
- Unix epoch = JD 2,440,587.5
Julian Day Format
Subtract 2,440,587.5 and multiply by 86,400,000 to get Unix milliseconds.
How do I convert a Julian Day Number to a calendar date?
Paste the Julian Day value into the converter above. It subtracts 2,440,587.5 (the JD value at the Unix epoch) and converts the remaining days to a Unix timestamp and readable date.
Frequently Asked Questions
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