Julian Day Converter

Julian Day is a running count of days with nothing else in it: no months, no leap years, no calendar reform, no time zones. Day zero begins at noon on 1 January 4713 BC in the proleptic Julian calendar, and every instant since has exactly one number. That is why astronomy and historical chronology use it — subtracting two dates is subtraction, and a date from before the Gregorian reform needs no special handling because the count never noticed the reform happened.

The converter goes both ways and emits the variants you are likely to need. JD itself starts at noon, which is a deliberate choice from before electric light: an astronomer working through the night should not have the date change halfway through the observation. MJD subtracts 2400000.5 precisely to undo that, moving the day boundary back to midnight for satellite tracking and telemetry, with its epoch at 17 November 1858. Truncated JD and RJD are further shifts for systems that wanted smaller numbers. The Unix epoch sits at JD 2440587.5, and the J2000.0 standard epoch is JD 2451545.0 — noon on 1 January 2000, not midnight, for the same reason.

The matching Julian-calendar date is given alongside the Gregorian one because the two disagree, and the size of the disagreement changes over time. A date written by a contemporary of Kepler is not the date a modern calendar assigns to that day, and the gap was ten days when the reform began and is thirteen now. If you are converting a historical record, the calendar it was written in matters more than the arithmetic.

One caveat if precision matters. A Julian Date is only as well-defined as the time scale behind it, and the common scales differ by a fixed but non-zero amount: Terrestrial Time currently runs about 69.2 seconds ahead of UTC, because TAI is 37 seconds ahead of UTC and TT is 32.184 seconds ahead of TAI. Ephemerides are published in TT while your clock reads UTC, so a JD copied from one context into the other is a minute-scale error — irrelevant for a date, fatal for an occultation timing.