📅

Julian Day Converter (Calendar Date ↔ JD, MJD & Julian Date Codes)

Convert a calendar date and time to the astronomical Julian Day (JD), Julian Day Number and Modified Julian Date — or a JD back to a date — plus the YYDDD Julian date code and the Julian-calendar date.

Convert

Date

Hour (UT, 0–23)

Minute

The date above is in the

Julian Day (JD)

Loading calculator…

How it works

JD = ⌊365.25 (Y + 4716)⌋ + ⌊30.6001 (M + 1)⌋ + D + B − 1524.5, with January/February counted as months 13/14 of the previous year and B = 2 − ⌊Y/100⌋ + ⌊Y/400⌋ for the Gregorian calendar (0 for Julian) · JDN = ⌊JD + 0.5⌋ · MJD = JD − 2,400,000.5 · YYDDD = 2-digit year + day of year

The Julian Day is a continuous count of days (and fractions) since noon Universal Time on January 1, 4713 BC in the Julian calendar, introduced by Joseph Scaliger and used by astronomers because it turns any date difference into a subtraction. A new JD begins at noon, so midnight dates end in .5. The Modified Julian Date starts at midnight and drops the leading 24 to keep numbers short. Separately, industry and the military use "Julian date" for a year-plus-day-of-year code (26277 = day 277 of 2026). The conversion uses Jean Meeus's algorithm from Astronomical Algorithms.

Worked example

January 1, 2000 at 12:00 UT (the J2000.0 epoch) is JD 2451545.0000, Julian Day Number 2451545 and MJD 51544.5000. It was a Saturday, day 1 of the year (code 00001), and December 19, 1999 in the Julian calendar.

Sources: Meeus, Astronomical Algorithms (2nd ed.), ch. 7 · US Naval Observatory — Julian Date converter

Frequently asked questions

What is today's Julian date?

It depends on which "Julian date" you mean. Astronomers mean the Julian Day, about 2.46 million in the 2020s (JD 2461317.5 is the start of October 4, 2026, UT). Food packers, the military and mainframes mean the YYDDD or YYYYDDD code — the year plus the day of the year, e.g. 26277 for October 4, 2026.

Why does the Julian Day start at noon?

So that a whole night of observing falls on one date for European astronomers. Midnight UT is therefore always X.5, and noon UT is a whole number.

What is the difference between the Julian Day and the Julian calendar?

Different things that share a name. The Julian calendar is Julius Caesar's calendar with a leap year every 4 years, now 13 days behind the Gregorian calendar. The Julian Day is a day count named after Scaliger's father, Julius Caesar Scaliger, and works with either calendar.

What is MJD?

The Modified Julian Date, JD − 2,400,000.5. It starts at midnight on November 17, 1858 and is used for satellites, spacecraft and many scientific data sets.

Related calculators

Ordinal Date (Day of Year) Calculator

Find the ordinal date — the day-of-year number from 1 to 366 — for any date, with days remaining and the percent of the year elapsed.

Day of the Year Calculator

Find which numbered day of the year any date is (today by default), how many days remain and how much of the year has passed — or turn a day number or a Julian date code like 26166 back into a date — with ISO week, ordinal date and Julian Day Number.

Unix Timestamp to Date Calculator

Convert a Unix timestamp in seconds or milliseconds to a readable UTC date and time, with the ISO 8601 string and weekday.

Days Since Unix Epoch Calculator

Count the whole days from the Unix epoch (January 1, 1970) to any date — the epoch day number, plus weeks and years elapsed.

Date Difference Calculator

Count the exact days between two dates — optionally including the end date — and see the gap in years, months and days, weeks, hours and minutes, plus working days with weekends and US, Mexican or Spanish holidays left out.

This calculator is for educational purposes. Double-check important results before acting on them.