Julian Date Calculator

1. Enter a calendar date, or leave it blank to use today in UTC.
2. Optionally set a UTC hour and minute. Leave both at 0 for midnight. The fractional Julian Date changes through the day.
3. Read the Julian Day Number (the astronomical count), the day of year (the ordinal date most people mean), and the extras below.

The phrase "Julian date" means two different things, and the calculator gives both so there is no confusion.

The Julian Day Number is an astronomical count of days that starts at noon Universal Time on January 1, 4713 BC in the proleptic Julian calendar. Astronomers use it to subtract any two dates without worrying about months, leap years or calendar reforms. The U.S. Naval Observatory defines it this way, and the Julian Date is simply that count plus the fraction of the day elapsed since the previous noon.

The ordinal date, also loosely called a "Julian date", is just the day of the year: 1 for January 1 and up to 365 or 366 for December 31. Supply chains, aviation, manufacturing and food packaging print it so a five-digit code such as 26279 means the 279th day of 2026. ISO 8601 calls this an ordinal date and writes it 2026-279.

If a label or form asks for a "Julian date" with three digits, it almost always wants the day of year, not the seven-digit astronomical number.

The calculator uses the standard integer algorithm published by Fliegel and Van Flandern in 1968. It turns a Gregorian year, month and day straight into the Julian Day Number with no loops.

1. Shift January and February to the end of the previous year with the term a, so leap days land correctly.
2. Add the regular 365.25 days per year through the 1461/4 term.
3. Add the days for whole months, then subtract the Gregorian century correction (skip three leap days every four centuries).
4. The result is the JDN at noon. For midnight subtract 0.5; for any UTC time add the fraction of the day since the previous noon.

The day of year is found the same way: the JDN of your date minus the JDN of January 1 that year, plus one. The Modified Julian Date is JD minus 2,400,000.5.

Take October 6, 2026 at midnight UTC.

Julian Day Number: the Fliegel-Van Flandern formula gives JDN = 2,461,320, the value at noon on that date. At 00:00 UTC the Julian Date is 2,461,320 - 0.5 = 2,461,319.5.

Day of year: January 31 + February 28 + March 31 + April 30 + May 31 + June 30 + July 31 + August 31 + September 30 = 273 days, plus 6 days into October = 279. Because 2026 is not a leap year the total is 365, so 86 days remain.

Check against J2000.0: the standard epoch 2000-01-01 12:00 UTC is exactly JD 2,451,545.0. From that noon to 2026-10-06 noon is 2,461,320 - 2,451,545 = 9,775 days, which matches a direct day count.

Only three things move the numbers: the date, the UTC time, and the calendar you assume. Local time zones do not, because the Julian Date is defined in UTC.

The Time of Day

The integer JDN is the same all day, but the fractional Julian Date rises steadily and crosses a whole number at noon UTC, not midnight. That half-day offset is the most common source of error.

Leap Years

A leap year adds February 29, so the day of year reaches 366 and every date after February 29 shifts up by one. The tool applies the Gregorian rule: every fourth year, except century years not divisible by 400.

Which Calendar

Input dates are read on the proleptic Gregorian calendar. Historical dates before each country adopted the Gregorian reform may differ by several days from old records that used the Julian calendar.

These two values share a name but answer different questions. Pick the one your task needs.

A packaging code like 6279 or 26279 is the ordinal form: day 279 of a year ending in 6. A value near 2.46 million is always the astronomical Julian Day Number.

Astronomy and Spaceflight

Observers log events in Julian Date so two measurements can be subtracted directly. Many data files use the Modified Julian Date to keep the numbers smaller and start the day at midnight.

Manufacturing and Food Packaging

Factories stamp the ordinal day of year as a lot or best-before code. Day 279 is cheaper to print and sort than a full date.

Aviation and Logistics

Flight and shipping records often carry the day of year, sometimes with the last digit of the year in front, as in 6279.

Date Arithmetic

To count the days between two far-apart dates, subtract their Julian Day Numbers. For everyday spans the date calculator and days from date calculator are simpler.

1. Mixing Up the Two Meanings

A three-digit "Julian date" on a carton is the day of year, not the seven-digit astronomical count. Reading one as the other is the biggest error.

2. Forgetting the Noon Start

The Julian day begins at noon UTC. Midnight is the JDN minus 0.5, so a date at 00:00 ends in .5, not .0.

3. Using Local Time

Julian Dates are UTC. Entering a local clock time without converting can shift the fraction, and near midnight even the day.

4. Ignoring Leap Years in the Day of Year

After February 29 a leap year pushes every ordinal number up by one and allows day 366.

5. Assuming the Same Calendar for Old Dates

Dates before the Gregorian reform may be recorded on the Julian calendar and differ by up to a couple of weeks.

The Fliegel-Van Flandern algorithm is exact for every date on the proleptic Gregorian calendar, so the integer results carry no rounding error. The fractional Julian Date is shown to five decimals, about one second.

For October 6, 2026 the astronomical Julian Day Number is 2,461,320 (2,461,319.5 at 00:00 UTC) and the ordinal day of year is 279. Leave the date blank to read today automatically.

Not quite. The day of year, 1 to 366, is often called a "Julian date" on packaging and forms. The astronomical Julian Day Number is a much larger continuous count. This tool shows both.

Because the Julian day starts at noon UTC. Midnight is half a day before the next whole number, so 00:00 UTC is the Julian Day Number minus 0.5.

The standard epoch 2000-01-01 at 12:00 UTC is exactly Julian Date 2,451,545.0. Astronomers use it as a reference point.

The Modified Julian Date is the Julian Date minus 2,400,000.5. It uses smaller numbers and starts each day at midnight instead of noon.

Read the last digit or two as the year and the final three digits as the day of year. So 26279 means day 279 of 2026, which is October 6.

Count forward from January 1. Day 279 of 2026 is October 6. Remember a leap year has an extra day after February 29.

The integer Fliegel-Van Flandern method from 1968, the standard way to turn a Gregorian date into a Julian Day Number without loops.

No. All times are Universal Time (UTC), matching the astronomical definition. Convert local time to UTC before entering an hour.

The Julian Day Number is defined for dates from its 4713 BC origin onward, but the date box in your browser does not accept BC years, so only AD dates can be entered.

• U.S. Naval Observatory: Julian Date (Astronomical Applications; the standard Julian Date definition and converter).
• Fliegel and Van Flandern, "A Machine Algorithm for Processing Calendar Dates" (1968) (Communications of the ACM 11(10):657; the integer algorithm used here).
• ISO 8601 date and time format (International Organization for Standardization; ordinal dates, written year-day).