Julian date converter
Astronomers mean the Julian Day Number; a continuous count of days from 4713 BC, currently around 2,460,000, used because subtracting two of them gives an interval with no calendar arithmetic at all. Manufacturing and aviation usually mean the ordinal date, a number from 1 to 366 within a year. They are entirely different things sharing a name, and this tool gives the astronomical one.
The Julian Day Number counts days continuously from noon UTC on 1 January 4713 BC. 1 January 2026 is JD 2461041.5. Because it is a plain running count, subtracting two Julian days gives an interval directly with no calendar arithmetic.
How to convert to a Julian day
The noon start is the detail that catches people. Julian days begin at noon UTC so that a single night of observation falls within one Julian day rather than straddling two, which was entirely sensible for astronomers and is confusing for everyone else. It is why a midnight date converts to a value ending in .5 rather than a whole number.
Questions
A continuous count of days from noon UTC on 1 January 4713 BC.
Because Julian days start at noon, so midnight falls half a day in.
No. Those are ordinal dates from 1 to 366 within a year: a completely different thing with the same name.
It is the point where three calendar cycles coincide, chosen so all recorded history has a positive number.
JD minus 2400000.5, which shortens the number and starts the day at midnight.