Julian Day Calculator

How to Use the Julian Day Calculator

1. 1. Choose what you have. Pick Calendar date and time to get a day count, or Julian Day or Modified Julian Day to get a date. The date form opens on the current UTC time.
2. 2. Enter the date. Type the year in astronomical numbering (1 BCE is year 0, 2 BCE is −1), pick the month, and give the day and the time of day in UTC. A blank time means midnight.
3. 3. Choose the calendar. Historical uses the Julian calendar before 15 October 1582 and the Gregorian one from then on, as astronomers do. Choose Gregorian or Julian to run one calendar throughout.
4. 4. Read the result. You get JD, MJD, the Julian Day Number, Unix time and the weekday, the date in both calendars, and the Reduced JD, Truncated JD and Rata Die values.

What a Julian Day Is

The Julian Day is a single count of days, with a decimal fraction, from noon Universal Time on 1 January 4713 BCE in the proleptic Julian calendar. Astronomers use it because it is continuous: the difference between two events is the number of days between them, with no months, leap years or calendar changes to untangle. The name comes from the Julian Period that Joseph Scaliger proposed in 1583, built on the Julian calendar, and the epoch is a Julian-calendar date.

The count starts at noon, so a Julian Day ends in .5 at civil midnight: midnight at the start of 1 January 2000 is JD 2451544.5, and noon that day is JD 2451545.0. The Julian Day Number (JDN) is the whole number that applies from one noon to the next, so it identifies the civil day that contains the noon — 2451545 for 1 January 2000.

The Calendar Question

The Julian Day count is unambiguous; the date you attach to it is not. Dates before 15 October 1582 were written in the Julian calendar, and the day after Thursday 4 October 1582 was Friday 15 October. The historical setting follows that convention, which is the convention in Jean Meeus's Astronomical Algorithms (2nd ed., Willmann-Bell, 1998), whose worked examples the tests reproduce. Dates 5–14 October 1582 do not exist in it, and the calculator says so. For a date in a source written in another calendar, convert it first with the Julian Calendar Converter.

Worked Examples

• J2000.0 — noon on 1 January 2000 — is JD 2451545.0, MJD 51544.5, a Saturday.
• The Unix epoch, 00:00 UTC on 1 January 1970, is JD 2440587.5 and MJD 40587.
• Unix time 1 000 000 000, 01:46:40 UTC on Sunday 9 September 2001, is JD 2452161.574074.
• JD 0 is 1 January 4713 BCE in the Julian calendar, which is 24 November 4714 BCE in the proleptic Gregorian calendar.
• 15 October 1582, the first Gregorian day, has Julian Day Number 2299161.

Accuracy and Limits

The day arithmetic is exact: the calculator uses the fixed-day algorithms of Reingold and Dershowitz, Calendrical Calculations: The Ultimate Edition (Cambridge University Press, 2018), and its tests check every example in Meeus's chapter and every 97th day of the whole range, 4713 BCE to 9999 CE, against Meeus's own formula. The Julian Day itself is shown to six decimals, about 0.09 seconds. What the number cannot tell you is the time scale. A JD computed from a UTC clock is on the UTC scale; JD in Terrestrial Time, the scale of J2000.0, is about 69 seconds ahead today, and the gap between Universal and Terrestrial time was hours in antiquity. Check which scale your ephemeris or paper uses.

Frequently Asked Questions

How do I convert a date to a Julian Day?

Count the days from the epoch, 1 January 4713 BCE at noon. By formula, take the Julian Day Number of the date (for the Gregorian calendar, 1 January 2000 is 2451545), subtract 0.5 and add the fraction of the day elapsed since midnight. The calculator does this for you and shows the whole-day number separately.

What is the difference between JD and MJD?

The Modified Julian Day is the Julian Day minus 2 400 000.5. That shifts the start of the day from noon to midnight and removes the leading digits, so a date in 2026 has an MJD near 61 300 rather than a JD near 2 461 300. The IAU recommended it in 1973 for satellite and radio work.

Why does the Julian Day start at noon?

Astronomers used to observe through the night, and a day count that changed at midnight would have changed in the middle of every night's observations. Counting from noon puts the change in the daytime, when no one was observing. The convention has been kept for historical consistency, even though most modern time scales start at midnight.

What is the Julian Day Number?

It is the integer part of the Julian Day measured from noon: the number of the civil day that contains that noon. Every moment from noon on 1 January 2000 to the next noon has the Julian Day Number 2451545. It is widely used as a plain day index, for instance in calendar conversion algorithms.

Is the Julian Day related to the Julian calendar?

Only through its epoch. The Julian Period was devised by Joseph Scaliger in 1583, and the epoch is a date in the Julian calendar, but the count itself is independent of any calendar. You can attach a Julian Day to a Gregorian, Julian or any other date.

Can I enter dates before year 1?

Yes, down to 1 January 4713 BCE in the Julian calendar. Use astronomical year numbering, in which 1 BCE is year 0, 2 BCE is year −1, and so on; the result shows the year as a BCE number as well. The historical calendar uses the Julian rules for all of these dates.