Julian Day Converter
Julian Day is astronomy's preferred timestamp — a continuous count of days since 4713 BC, no leap years or calendar reforms to worry about. This converter walks both directions: pick a Gregorian date + time to get the JD, or paste a JD to see when that is. It also emits the common shifts — JDN (rounded to noon), MJD (JD − 2400000.5, the standard for satellite tracking), RJD, Truncated JD, the matching Julian-calendar date, the Unix timestamp, weekday and day-of-year — so whichever flavour your downstream system speaks, you have it.
| Julian Day (JD) | 2461273.500000 | |
| Julian Day Number (JDN) | 2461274 | |
| Modified Julian Day (MJD) | 61273.000000 | |
| Reduced Julian Day (RJD) | 61273.500000 | |
| Truncated Julian Day (TJD) | 21273.000000 | |
| Unix timestamp | 1787270400 | |
| Gregorian date | 2026-08-21 00:00:00 UTC | |
| Julian-calendar date | 2026-08-08 | |
| Weekday | Friday | |
| Day of year | 233 |
Conversion uses the standard Meeus algorithm. Valid for dates from JD 0 (4713 BC) through the far future — leap-year rules and the 1582 Gregorian reform are both handled.
How to use
- Pick the direction. Date → JD takes a Gregorian date and time; JD → date takes a Julian Day number.
- Read the row that matches your downstream system — astronomy and satellite tools usually want MJD; observatory logs and ephemerides use full JD.
- Copy the value you need. The Unix timestamp helps when bridging to standard programming APIs.
Frequently asked questions
- Why does a Julian Day include a `.5`?
- JD epochs start at noon UT, not midnight. So 2026-01-01 00:00 UT is JD 2461041.5. The half is what aligns 'days' to start when astronomers' observations begin.
- Julian Day vs Julian calendar?
- Totally different things despite the name. Julian Day is a continuous day count for time calculations. Julian calendar is the pre-1582 calendar (currently 13 days behind Gregorian). This tool shows both because the second is what historical astronomers used to record observations.
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