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.

Julian Day Converter

Time

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.

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Gregorian date (UTC)Time (UTC)

Results

Julian Day (JD)	2461201.500000
Julian Day Number (JDN)	2461202
Modified Julian Day (MJD)	61201.000000
Reduced Julian Day (RJD)	61201.500000
Truncated Julian Day (TJD)	21201.000000
Unix timestamp	1781049600
Gregorian date	2026-06-10 00:00:00 UTC
Julian-calendar date	2026-05-28
Weekday	Wednesday
Day of year	161

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.