Julian Day Number Converter
Convert any Gregorian calendar date to its astronomical Julian Day Number and back — day 0 is noon, 24 November 4714 BC.
61347.500000
Tuesday
2026 is a common year in this calendar — 365 days.
| Reduced JD (JD − 2400000) | 61348.000000 | |
|---|---|---|
| Truncated JD (JD − 2440000.5) | 21347.500000 | |
| Days since J2000.0 (JD − 2451545) | 9803.000000 | |
| Unix time (seconds) | 1793707200 | |
| Ordinal date (YYDDD · YYYYDDD) | 26307 · 2026307 |
Show the working (Fliegel–Van Flandern)
Counting days since 4714 BC
This converter turns any Gregorian calendar date into its Julian Day Number (JDN) and turns any Julian Day Number back into a calendar date. The Julian Day Number is a simple running count of whole days that astronomers use to avoid the messiness of months, leap years, and calendar reforms: day 0 is fixed at noon on 24 November 4714 BC in the proleptic Gregorian calendar, and every day since then just adds one. Pick a date or type a number and the other field updates immediately; a date picker cannot hold a year before 1 AD, so an AD/BC choice beside it lets you enter BC dates as well. A handy checkpoint is the J2000.0 epoch — noon on 1 January 2000 — which has the round Julian Day Number 2451545.
Why the Julian Day Number is useful
Because a JDN is just an integer, the number of days between two events is a plain subtraction, with no need to worry about how many days each month has or whether a leap year sits in between. That makes it the standard timestamp for astronomical observations, satellite tracking, historical chronology, and any software that needs to compare or sort dates spanning many centuries. This tool uses the classic Fliegel–Van Flandern integer algorithm, so the result is exact and identical every time. Everything is computed locally in your browser, so you can convert as many dates as you like without limits.
Frequently asked questions
What is the Julian Day Number for 1 January 2000?
Noon on 1 January 2000 — the J2000.0 astronomical epoch — has Julian Day Number 2451545. This tool returns that value when you enter 2000-01-01, and returns 2000-01-01 when you enter 2451545.
Is the Julian Day Number the same as the Julian calendar?
No. Despite the shared name, the Julian Day Number is a continuous count of days used in astronomy, while the Julian calendar is an older calendar system replaced by the Gregorian calendar in 1582. This tool converts to and from Gregorian dates by default, and can read the date as a Julian-calendar one instead — or as Julian before the 1582 reform and Gregorian after it.
Can I include a time of day, or use the Julian calendar?
Yes. Add a time and the Julian Date gets its fraction of a day; the Julian Date can be typed back too, and the Modified Julian Date and weekday are shown. The date can be read as Gregorian, as Julian, or as Julian before the Gregorian reform, and an AD/BC choice takes BC dates.
Can it read an MJD or a Unix time, and show the formula?
Yes. “The same moment on other scales” restates the day and time as days since J2000.0, Reduced JD, Truncated JD and Unix seconds, each with its own copy button, and adds the ordinal date (YYDDD · YYYYDDD) — what “Julian date” means outside astronomy. “Or type a value on another scale” takes an MJD, a Reduced or Truncated JD, days since J2000.0 or a Unix time and moves the day, date and time to it. “Show the working (Fliegel–Van Flandern)” writes out every term of the day-number sum with this date's own numbers.
Does it say whether the year is a leap year?
Yes. A line under the weekday says whether the year is a leap or a common year in the calendar the date is read in, and how many days it has from 1 January to 1 January. The calendars disagree about century years — 1900 is common in the Gregorian calendar and leap in the Julian — the reform year 1582 is cut short by the days the reform dropped, and a BC year is named with its era.