Julian Date Converter
Convert between Gregorian calendar dates and Julian dates (ordinal date format). Julian date format uses yyddd or yyyyddd where yy/yyyy is the year and ddd is the day of year.
Convert Gregorian Date to Julian Date
Enter a standard calendar date to get the corresponding Julian date in yyddd or yyyyddd format
📅 Input Date
Common Use Cases & Examples
Product/Batch Code Decoding
Who: Consumers, Retail/Warehouse Staff
24123 = May 2, 202423365 = Dec 31, 2023Data Format Conversion
Who: Programmers, Data Analysts, Excel Users
01/15/2024 → 202401512/31/2023 → 2023365Industrial/Logistics
Who: Manufacturing, Supply Chain Professionals
Batch: 25170LOT-25170-A1About Julian Dates (Ordinal Format)
We refer to a yyddd date format (yy = year, ddd = day) as a 'Julian Date' - this is the common term for such a date in mainframe and other computing circles. However technically, a Julian date can mean different things. Astronomers refer to a Julian date as the number of days since the beginning of the Julian Period (January 1, 4713 BC). A yyddd (or similar format such as yyyyddd, yy-ddd) date is more correctly called an ordinal date. However in the mainframe world, we call them 'Julian Dates'.
Format Specifications:
- yyddd: 5-digit format (YY + day of year)
- yyyyddd: 7-digit format (YYYY + day of year)
- Day of Year: 001-365 (366 for leap years)
- Year Interpretation: For yyddd, 00-49 = 20xx, 50-99 = 19xx
Example Conversions:
- January 1, 2024 = 24001 (yyddd) or 2024001 (yyyyddd)
- December 31, 2023 = 23365 (yyddd) or 2023365 (yyyyddd)
- February 29, 2024 = 24060 (yyddd) or 2024060 (yyyyddd) - leap year
The Julian date (ordinal format) is calculated using the following steps:
// Calculate day of year
startOfYear = new Date(year, 0, 1)
targetDate = new Date(year, month-1, day)
dayOfYear = floor((targetDate - startOfYear) / (24*60*60*1000)) + 1
// Format as Julian date
yyddd = (year % 100).toString().padStart(2,'0') + dayOfYear.toString().padStart(3,'0')
yyyyddd = year.toString() + dayOfYear.toString().padStart(3,'0')Day of year ranges from 001 to 365 (or 366 in leap years). For yyddd format, years 00-49 are interpreted as 20xx, and 50-99 as 19xx.
Mainframe & Legacy Systems:
- IBM mainframe date storage and processing
- COBOL and PL/I date calculations
- Legacy system integration and migration
- Batch processing and job scheduling
Manufacturing & Logistics:
- Production planning and scheduling
- Inventory management systems
- Quality control and expiration tracking
- Supply chain date calculations
Data Processing & Analysis:
- Sequential date processing in databases
- Time series analysis with ordinal dates
- Data migration and format conversion
- Report generation and data extraction
Standard yyddd (5-digit)
Format: YYDDD (e.g., 24015)
Most common in legacy mainframe systems. Year interpretation: 00-49 = 20xx, 50-99 = 19xx.
Extended yyyyddd (7-digit)
Format: YYYYDDD (e.g., 2024015)
Preferred for modern systems to avoid Y2K-style issues. Explicit 4-digit year eliminates ambiguity.
Delimited Format (yy-ddd)
Format: YY-DDD (e.g., 24-015)
Sometimes used for readability in reports and displays, though less common in data storage.
📅 What's the difference between Julian Date and Julian Day Number?
Julian Date (yyddd format) is an ordinal date used in mainframes, while Julian Day Number is an astronomical system counting days since 4713 BC. They serve different purposes despite similar names.
🔢 Should I use yyddd or yyyyddd format?
Use yyyyddd for clarity and Y2K compliance. The yyddd format assumes 00-49 = 20xx and 50-99 = 19xx, which may cause confusion near century boundaries.
📊 How do I handle leap years?
Leap years have 366 days instead of 365. February 29th becomes day 060, and all subsequent days shift by one. Our converter automatically handles leap year calculations.
🏢 Why is this called Julian Date in mainframes?
Historical naming convention in the mainframe world. Though technically "ordinal date" is more accurate, "Julian Date" became the standard term in COBOL, PL/I, and other mainframe environments.
🗓️ What's the range of valid dates?
For yyddd: 1950-2049 (50001-49365). For yyyyddd: any valid Gregorian year. Day of year ranges from 001-365 (366 in leap years).
⚙️ Can I use this for date arithmetic?
Yes, but be careful with year boundaries. Adding days to day 365 might roll over to the next year. For complex calculations, convert to Gregorian dates first.