What is a Julian Day Number?

A continuous count of days since noon on 1 January 4713 BC in the proleptic Julian calendar. It gives every day a single integer with no months or leap years involved, which makes date arithmetic trivial for astronomers.

Why does the Julian day start at noon?

Because it was designed for astronomers, who observe at night and would rather not have the date change in the middle of a session. The Modified Julian Date shifts it back to midnight for everyone else.

What is the Modified Julian Date?

The Julian Date minus 2400000.5, which puts day zero at midnight on 17 November 1858 and makes the number small enough to be readable. Space agencies and geodesy use it heavily.

What does a Julian date on food packaging mean?

It is the ordinal form, not the astronomical one: the last digits are the day of the year, so 26060 is the 60th day of 2026. Manufacturers use it because it sorts cleanly and fits in five characters.

How do I get today's Julian date?

Use the now button, which stamps the current UTC moment and fills in every form. The whole number part is the day and the fraction is the time since the previous noon.

Julian Date Converter

Convert calendar dates to the astronomical Julian Day and to the ordinal YYDDD form, both ways.

The Julian Date Converter runs entirely in your browser. Dates are converted on your device and nothing is sent anywhere.

Convert a Unix timestamp

About Julian Date Converter

Two quite different things are called a Julian date, and the confusion between them is exactly why this tool exists. The astronomical Julian Day is a continuous count of days since 4713 BC, used in astronomy and satellite work because it has no months, no leap years and no time zones to argue about — and because it starts at noon, not midnight. The other is the ordinal date stamped on packaging, in aviation logs and in mainframe files: a year followed by the day of the year, so the first of March 2026 is 26060. This converter handles both, in both directions, along with Modified Julian Date and the day of the week.

Features

Calendar date to Julian Day and back, including the fraction of a day
Modified Julian Date and Reduced Julian Date alongside the full figure
Ordinal YYDDD and YYYYDDD forms, read and written
Day of the year, day of the week and leap-year status
Days since the J2000.0 epoch, for astronomical work
Unix timestamp for the same instant
Everything computed in UTC, so a local time zone cannot shift the day

How to use the Julian Date Converter

Pick whether you are starting from a calendar date, a Julian Date or an ordinal date
Enter the value, adding a time of day if you need the fraction
Read every other form in the results table
Use the now button to stamp the current UTC moment

Example

Input

2000-01-01 12:00 UTC

Output

JD 2451545.0 · MJD 51544.5 · ordinal 00001

J2000.0, the reference epoch for modern astronomical coordinates.

Common errors & troubleshooting

The Julian Date is half a day off what you expected. — The Julian day starts at noon, not midnight. Midnight UTC is always a .5 value, so 2451545.0 is midday on 1 January 2000 and 2451544.5 is the midnight that began it.
An ordinal date from a product label will not parse. — It needs five digits (YYDDD) or seven (YYYYDDD). Some manufacturers add a plant or shift code around it, so strip anything that is not the year and the day number.
A two-digit ordinal year lands in the wrong century. — Years 00 to 69 are read as 2000s and 70 to 99 as 1900s, which is the usual pivot. Use the seven-digit form when the century has to be unambiguous.
The date is a day out against a local calendar. — Everything here is UTC. If your source date was local and near midnight, convert it to UTC first — that is exactly the ambiguity Julian Dates exist to remove.

Frequently asked questions

What is a Julian Day Number?: A continuous count of days since noon on 1 January 4713 BC in the proleptic Julian calendar. It gives every day a single integer with no months or leap years involved, which makes date arithmetic trivial for astronomers.
Why does the Julian day start at noon?: Because it was designed for astronomers, who observe at night and would rather not have the date change in the middle of a session. The Modified Julian Date shifts it back to midnight for everyone else.
What is the Modified Julian Date?: The Julian Date minus 2400000.5, which puts day zero at midnight on 17 November 1858 and makes the number small enough to be readable. Space agencies and geodesy use it heavily.
What does a Julian date on food packaging mean?: It is the ordinal form, not the astronomical one: the last digits are the day of the year, so 26060 is the 60th day of 2026. Manufacturers use it because it sorts cleanly and fits in five characters.
How do I get today's Julian date?: Use the now button, which stamps the current UTC moment and fills in every form. The whole number part is the day and the fraction is the time since the previous noon.

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