Julian Date Calculator / Converter (Accurate Results)

Whether you spotted a strange number on a food package, need to do precise astronomical calculations, or are decoding a military MRE stamp, you’ve come to the right place. Use the Julian date calculator below to instantly convert any calendar date to a Julian Day Number and vice versa.

What Is a Julian Date?

A Julian Date (JD) is a continuous count of days (and fractional days) elapsed since noon Universal Time on January 1, 4713 BC in the proleptic Julian calendar.

Rather than counting within months and years, which vary in length and have changed across history, the Julian Date system counts forward from a single fixed point in time, making it ideal for any calculation that spans long time periods.

Today’s Julian Day Number is displayed automatically in the julian date calculator above. It updates in real time.

One key detail: the Julian Day begins at noon UTC, not midnight. So a date like January 1, 2000, at midnight corresponds to JD 2,451,544.5, while noon on the same day is the more commonly cited JD 2,451,545.0.

How to Use This Julian Date Calculator

The tool includes four modes:

1. Date → Julian: Enter any calendar date and optional UTC to get the Julian Day Number, MJD, Truncated JD, J2000.0 offset, Unix timestamp, and day of the week. Pre-filled with today’s date and current time automatically.
2. Julian → Date: Enter any Julian Day Number to convert it back to a full calendar date and time, including ISO 8601 format.
3. Date Difference: Enter two calendar dates to calculate the exact number of days between them, using Julian Day arithmetic for precision.
4. Quick Reference: A reference table of important epochs, events, and derived time scale formulas.

Results can be copied to the clipboard with one click.

How to Read a Julian Date on Food Packaging

When you see a numeric code on a can, carton, or packaged good, it is almost certainly the ordinal Julian date format, not the astronomical JDN.

Here’s how to decode the most common formats:

YYDDD (5-Digit Format) – Most Common

The first two digits are the last two digits of the year; the final three digits are the day of the year.

YYYYDDD (7-Digit Format) – Pharmaceutical & Military

Used when long-term traceability is required. The first four digits are the full year; the last three are the day.

YDDD (4-Digit Format) – Military MREs

Used on US military Meals Ready-to-Eat (MRE). Only the last digit of the year is used, since MREs are not expected to last beyond 10 years.

Important note: The Julian date on food packaging is a production date, not an expiration date.

How to Calculate a Julian Day Number

You don’t need to do this by hand; the Julian date calculator at the top handles it instantly.

But if you’re curious about the underlying algorithm, here is the standard formula for converting a Gregorian calendar date to a Julian Day Number, based on the method from Jean Meeus’s Astronomical Algorithms:

Step 1: Express your date as Year (Y), Month (M), Day (D).
Step 2: If M ≤ 2, set Y = Y − 1 and M = M + 12.
Step 3: Calculate A = floor(Y / 100) and B = 2 − A + floor(A / 4).
Step 4: JD = floor(365.25 × (Y + 4716)) + floor(30.6001 × (M + 1)) + D + B − 1524.5

For a Julian calendar date (historical, before October 15, 1582), the same formula applies but with B = 0.

Example: January 1, 2000 at noon UT
→ JD = 2,451,545.0 (the J2000.0 epoch)

Key Julian Date Epochs & Reference Points

These are the most important fixed points in the Julian Date system, used across astronomy, computing, and space science:

Modified Julian Date (MJD) Explained

The Modified Julian Date (MJD) was introduced by the Smithsonian Astrophysical Observatory in 1957 to track Sputnik’s orbit.

Because full Julian Day Numbers had grown into the 2.4 million range by the mid-20th century, scientists wanted a shorter number that was easier to work with in computations and data storage.

Formula: MJD = JD − 2,400,000.5

Two things have changed compared to the standard JD:

1. The number is roughly 2.4 million smaller (today’s MJD is around 60,000)
2. The day starts at midnight UTC rather than noon, making it more intuitive for modern use

MJD is widely used in satellite tracking, geodesy, and time standard agencies such as the International Earth Rotation and Reference Systems Service (IERS).

Other Derived Time Scales

The Julian Calendar vs. Julian Dates: Clearing Up the Confusion

These are two entirely separate things that share a name and the confusion is extremely common.

Key differences at a glance:

Frequently Asked Questions

Final Words!

Use the Julian date calculator/converter to get accurate results.

Calculations use the algorithm from Jean Meeus, Astronomical Algorithms (2nd ed.), Chapter 7. The Gregorian calendar reform date of October 15, 1582 (JD 2,299,161) is used as the cutoff between calendar systems.