Julian date converter, JD, MJD and back.

A Julian date converter turns a calendar date and time into the Julian date (JD), a single count of days that astronomers use to subtract one moment from another without thinking about months, leap years or calendars. It also turns a JD, a Modified Julian Date, a Unix time or a five digit "Julian date" from a food label back into a date. The top of this page shows the Julian date of right now, so it doubles as the Julian date of today.

How to use the converter

1. Under I have, choose what you are starting from: A calendar date and time, a JD, an MJD, Unix time or an ordinal date.
2. For a date, type the Year, Month and Day, set the Time (UTC), and pick the Calendar. Use this moment (UTC) fills in the present.
3. For a number, type it in the box that appears. A five digit ordinal date needs the century, because 26297 does not say whether it is 1926 or 2026.
4. Read the cards. The big one is the JD. Copy result copies the whole answer on one line, Copy JD copies only the number, and Copy link opens this page with the same inputs.

Three different things called a Julian date

The name is used for three separate things, which is why searches for it are confusing.

• The Julian date (JD) is the astronomers’ count of days, with a fraction for the time of day. It starts at noon UTC on 1 January 4713 BC in the Julian calendar. Its whole number is the Julian day number (JDN).
• The Modified Julian Date (MJD) is JD minus 2,400,000.5. It starts at midnight, so the numbers are shorter and a new day begins at .0 instead of .5.
• The ordinal "Julian date" on food, military and factory labels is just the day of the year, written as YYDDD or YYYYDDD. 26297 is day 297 of 2026, which is 24 October.

On top of that, the Julian calendar is the old calendar that Julius Caesar introduced and that the Gregorian calendar replaced. The converter shows the date in both calendars for every answer, and lets you type a date in either one.

Worked example

Take noon UTC on 1 January 2000. Count the days since the Unix start (1 January 1970): that is 10,957 days. Add the Unix start's JD of 2,440,587.5 and the midnight total is 2,451,544.5. Add 0.5 for the noon. That gives 2,451,544.5 for midnight, plus 0.5 for the noon. Put simply, 2000-01-01 at 12:00 is JD 2451545.0, and at midnight that morning it was 2451544.5. The MJD is 2451545.0 minus 2400000.5, or 51544.5.

Now the label example. A date of 24 October 2026 is day 297 of the year, so its YYDDD is 26297 and its YYYYDDD is 2026297. The Julian day number for that calendar day is 2461338, and midnight at the start of it is JD 2461337.5.

Reference moments

These come from this converter and were checked against an independent date library.

Notice that a JD ends in .5 at midnight and .0 at noon. That is the half-day offset behind many wrong answers. If a result looks half a day out, check whether the number was meant for midnight (MJD style) or noon (JD style).

The Julian and Gregorian calendars

Dates are shown in the proleptic Gregorian calendar, meaning the Gregorian rules are applied to dates before they were adopted. The Julian calendar date is worked from the same day number. In this converter, 4 October 1582 in the Julian calendar and 15 October 1582 in the Gregorian are consecutive days (JDN 2299160 and 2299161), which is why the Gregorian calendar started on 15 October. Today the two calendars are 13 days apart, so 24 October 2026 Gregorian is 11 October 2026 Julian.

What this does not do

This counts a plain 86,400 seconds in every day. It does not add leap seconds and does not convert between time scales such as UTC, UT1 and Terrestrial Time. Those differ by seconds, so the decimals on a JD show the arithmetic, not the accuracy of a clock. NASA's SPICE documentation also warns that the label JD alone is ambiguous: it can mean a count in UTC or in an ephemeris time scale (NASA NAIF time required reading, checked 24 Oct 2026). Say which scale you mean when you share a JD.

For the day of the year and the ISO week of any date, use the day of the year calculator. To convert Unix time on its own, try the Unix timestamp converter.