Julian day converter
Convert a UTC date and time to a Julian day and back.
| Date | October 16, 2026 at 11:54 PM UTC |
|---|---|
| Julian day | 2461330.49586 |
About this tool
The Julian day is a continuous count of days used by astronomers to avoid calendars altogether: day 0 began at noon on 1 January 4713 BC of the proleptic Julian calendar, and the count has simply gone on since. Each Julian day starts at 12:00 UTC, not at midnight, so that a whole night of observation falls on the same day number. J2000.0, the standard reference epoch, is JD 2451545.0 — 1 January 2000 at 12:00.
The conversion between a calendar date and a Julian day is exact; Explorer uses the classic algorithm from Jean Meeus for the Gregorian calendar and works in UTC. Astronomical formulas strictly use Terrestrial Time, which runs about 69 seconds ahead of UTC today — a difference with no visible effect on solar ephemerides at the precision of this site.
You will meet Julian days in the ephemeris tool, on observation pages and in the open data of the site: they make it trivial to compute intervals between observations or to feed a series into a spreadsheet. Enter a date and time to get its Julian day, or a Julian day to get the calendar date back.