Introduction
A Julian date is a way to write any calendar date as a simple number. Instead of using a month and day, it counts the days in order from the start of the year — or from a fixed point in history. Scientists, farmers, food makers, and shipping companies all use Julian dates because they make it easy to track time and count the days between two events.
This Julian Date Calculator lets you convert dates in four ways. The Quick Converter reads almost any date format you type and instantly gives you the ordinal Julian date (the day-of-year number) or turns an ordinal code back into a regular calendar date. The Date → JDN tab converts any calendar date into an astronomical Julian Day Number — the continuous count of days since January 1, 4713 BC — and shows each math step so you can follow along. The JDN → Date tab does the reverse, turning a Julian Day Number back into a calendar date and time. The Date Interval tab finds the exact number of days between two dates using their Julian Day Numbers.
All results are calculated in UTC. The tool supports both the Julian and Gregorian calendar systems and handles dates across thousands of years. Enter a date below to get started.
How to Use Our Julian Date Calculator
Enter a calendar date, an ordinal Julian date, or a Julian Day Number and this calculator will convert it for you. It can also find the number of days between two dates. Results include the Julian Day Number (JDN), ordinal Julian date, Modified Julian Date (MJD), and the day of the week.
Quick Converter Tab: Type any date into the input box. You can use formats like 07/11/2026, 11-Jul-2026, July 11 2026, or an ordinal code like 26192 or 2026-192. You can also type "today," "+1 week," or "-3 days" for quick relative dates. Press "Convert" to see the calendar date, ordinal Julian date, day of the week, and a chart showing how far through the year that date falls. If you need to add a specific number of days to a starting date, the Date Interval tab or our dedicated tool can help.
Date → JDN Tab: Pick an era (AD or BC), then enter a year, month, and day. Set the time in HH:MM:SS.S format if you need a precise result, or leave it at 00:00:00.0 for midnight. Choose "Auto" for the calendar system to let the tool switch between Julian and Gregorian at the 1582 reform, or pick "Julian Only" to force the Julian calendar. Press "Convert" to get the Julian Day Number, Modified Julian Date, day of the week, and a full step-by-step solution.
JDN → Date Tab: Enter a Julian Day Number in the input box. Decimals up to six places are accepted for a time-of-day result. Choose your calendar system, then press "Convert." The tool will return the matching calendar date, time in UTC, day of the week, and Modified Julian Date. If you need to work across different time zones, remember to convert your local time to UTC before entering it.
Date Interval Tab: Fill in a date in Panel A using the era, year, month, and day fields. Then either enter a date in Panel B or type a number of days in the "Days Later" box. Press "Calculate" and the tool will find the JDN for both dates and show the exact number of days between them. A positive interval means Panel B is after Panel A. A negative number means Panel B is before Panel A. For everyday interval counting that does not require Julian Day Numbers, you can also use our Days From Date Calculator.
What Is a Julian Date?
A Julian date is a way to write a calendar date as a simple number. There are two main types, and they are used for very different reasons.
Ordinal Julian Date (Day of the Year)
An ordinal Julian date counts how many days have passed in the current year. January 1 is day 1, February 1 is day 32, and so on up to day 365 (or 366 in a leap year). It is often written as a 5-digit code like 26192, where "26" is the year and "192" is the day number. You will see this format on food packages, medicine labels, and shipping boxes. It tells you when a product was made without using a full date. To find how many days have passed since a known production date, try our Days Since Calculator.
Julian Day Number (JDN)
A Julian Day Number is a running count of days that started at noon on January 1, 4713 BC. Today's JDN is in the millions. Scientists, astronomers, and historians use it because it gives every day in history one single number. This makes it easy to find the exact number of days between any two dates, even dates thousands of years apart. Our Age Calculator uses a similar principle to measure the time elapsed between two calendar dates. The Modified Julian Date (MJD) is a shorter version. You get it by subtracting 2,400,000.5 from the JDN.
Julian Calendar vs. Gregorian Calendar
The Julian calendar was created by Julius Caesar in 46 BC. It used a simple leap year rule: one leap year every 4 years. Over time, this added too many leap days, and the calendar drifted away from the real seasons. In 1582, Pope Gregory XIII fixed this by removing 10 days (October 5–14 were skipped) and adding a better leap year rule. This new system is the Gregorian calendar, which most of the world uses today. When you work with dates before October 15, 1582, the Julian calendar applies. After that date, the Gregorian calendar applies. You can explore more everyday date calculations with our Date Calculator.
How to Convert a Date to a Julian Day Number
The conversion uses a math formula that accounts for years, months, and the calendar system. First, you adjust the month and year. If the month is January or February, you subtract 1 from the year and add 12 to the month. Next, you calculate a correction term if the date falls in the Gregorian calendar. Finally, you plug everything into the formula to get the JDN. The calculator on this page does all of this work for you instantly.
Common Uses of Julian Dates
- Food and medicine: Lot codes on packaging often use the ordinal format (yyddd) to mark production dates.
- Astronomy: Astronomers use JDN to time events like eclipses, planet positions, and satellite passes.
- Military and government: The U.S. military uses ordinal dates in reports and logistics, often alongside military time for precise record-keeping.
- Computing: Some databases store dates as Julian Day Numbers because a single number is easier to sort and compare.
- Logistics and shipping: Companies use ordinal Julian dates alongside lead time calculations and business day counts to schedule production and delivery.