Julian Day Converter

Convert freely between a calendar date and its Julian Day Number, a single continuously incrementing integer used in astronomy and scientific computing, using the standard Gregorian-calendar conversion formulas in both directions.

What is the Julian Day Converter?

The Julian Day Number, or JDN, counts complete days since noon UTC on 1 January 4713 BC in the proleptic Julian calendar. Unlike a calendar date, it never resets: it is one continuously growing integer, currently seven digits long, which makes it convenient for date arithmetic in astronomy and some scientific and database systems. This page converts in both directions using the standard integer formulas, so you can turn any calendar date into its JDN, or any JDN back into the date it represents.

Key Features

• Calendar date to Julian Day Number conversion using the standard integer algorithm
• Reverse conversion from a Julian Day Number back to a calendar date
• Both directions run independently, so you can use either one on its own
• Quick-fill example buttons for well-known reference dates and JDNs
• A caption explaining the noon-UTC anchor point that JDNs are built around

Common Use Cases

• Converting a date into the JDN format expected by an astronomy tool or dataset
• Decoding a bare Julian Day Number found in a scientific file or database record
• Checking date arithmetic that relies on Julian Day Numbers under the hood
• Verifying a manual JDN calculation against a known reference date

How to Use the Julian Day Converter

1. In section A, pick a calendar date to see its Julian Day Number appear instantly below.
2. Use the example buttons to jump straight to well-known reference dates.
3. In section B, type an integer Julian Day Number to see the calendar date it represents.
4. The two sections work independently, so you can use either direction without touching the other.
5. Compare your own manual calculation against the quick-fill examples to sanity-check the formula.

Tips for Best Results

• Section B expects a whole number; fractional Julian Dates that include a time of day are not supported here.
• Dates are projected backward using modern Gregorian rules, so very old JDNs will not match historical calendars that used different systems at the time.
• 1 January 2000 is JDN 2451545, a useful number to remember for a quick sanity check.
• The JDN grows by exactly 1 for every calendar day that passes, with no leap-year irregularities to account for.

Why Use WiserWork's Julian Day Converter?

Working out a Julian Day Number by hand means running a multi-step integer formula correctly, and getting the reverse conversion right is just as fiddly. Both directions are easy to get subtly wrong through a single misplaced floor division, which is exactly the kind of arithmetic this page is built to run instantly and correctly every time.

Who Uses the Julian Day Converter?

Astronomers and students working with observation logs, developers debugging code or database records that store dates as Julian Day Numbers, and anyone who has come across a bare JDN in a scientific dataset and needs to know what calendar date it actually represents.

Frequently Asked Questions

Why does the Julian Day Number start at noon UTC instead of midnight?

It's a convention inherited from astronomy. Julian Day 0 begins at 12:00 UTC on 1 January 4713 BC (proleptic Julian calendar) so that a single night of observations, which often crosses local midnight, keeps the same day number from dusk until dawn instead of flipping over partway through.

Is this the same as the "day of year" ordinal date used elsewhere on this site?

No, and the two are easy to confuse. The three-digit ordinal date on WiserWork's Day of Year Calculator restarts at 1 every January and only ever goes up to 365 or 366. The Julian Day Number here is a single continuous count that has been running since 4713 BC and is currently in the seven-digit range; it never resets.

What's the practical use of Julian Day Numbers today?

Astronomers still use them to timestamp observations without worrying about calendar irregularities, and they show up under the hood in scientific computing and some database and spreadsheet systems, because a single incrementing integer makes date arithmetic, like finding the number of days between two dates, trivial compared to juggling months and leap years.

Two directions, one formula pair: this page runs the standard integer conversion both ways so you never have to work either one out by hand.