Convert a date to a Julian day number

The job

Ephemeris tables, satellite passes, and scientific datasets stamp events with a Julian day — a continuous count of days since 4713 BC — instead of year-month-day. Converting to and from a calendar date is arithmetic nobody wants to do by hand.

Convert

Enter a calendar date (and optional time, UTC) to get the Julian day and the modified Julian day (MJD = JD − 2,400,000.5, which starts at midnight rather than noon). Or enter a JD/MJD to get the matching calendar date back.

• JD changes at noon UTC — a 09:00 observation belongs to the 'previous' JD integer.
• MJD is what most data files use; JD is what astronomy tables print.

Worked example

2000-01-01 12:00 UTC is the famous epoch JD 2451545.0. The same instant as MJD is 51544.5 — the .5 marks the half-day offset between the two conventions.

Where it stops

Proleptic Gregorian calendar in UTC — no historical Julian-calendar conversion, no timezone offset, no leap seconds. 'Julian date' in logistics (yyddd day-of-year) is a different, unrelated format.