Day of Year Calculator
Convert any date to its day number in the year and discover seasonal insights
Tip: Use this for programming (Julian dates), project tracking, or astronomical calculations.
Date Conversion Results
Selected Date:
Day of Year:
Days Remaining:
Year Progress:
-
-
-
-
Seasonal Information
-
Programming Formats
ISO Ordinal Date:
Excel Serial Date:
-
-
Understanding Day of Year Calculations
The day of year (DOY) is a numerical representation ranging from 1 through 365 (or 366 in leap years) that indicates the day's position within the calendar year. This system, also known as the ordinal date, is widely used in:
Scientific Applications
- Astronomical observations
- Meteorological data tracking
- Ecological studies
- Satellite telemetry
Technical Uses
- Database timestamping
- File version control
- Project management
- Financial calculations
Leap Year Considerations
Our calculator automatically adjusts for leap years (years divisible by 4, except for years divisible by 100 unless also divisible by 400). February 29th will be correctly calculated as day 60 in leap years and ignored in non-leap years.
Key Day of Year Milestones
| Day Number | Regular Year Date | Leap Year Date | Significance |
|---|---|---|---|
| 1 | January 1 | January 1 | New Year's Day |
| 60 | March 1 | February 29 | Leap day (if applicable) |
| 91 | April 1 | March 31 | End of Q1 |
| 182 | July 1 | June 30 | Midpoint of year |
| 365 | December 31 | - | Year end (regular year) |
Note: In leap years, all days after February 28th have a day number one greater than in regular years.
Frequently Asked Questions
What's the difference between day of year and Julian date?
While often confused, day of year (1-365/366) is different from Julian dates which typically combine the year with day number (e.g., 2023150 for May 30, 2023). Astronomical Julian dates are a continuous count of days since 4713 BC.
How is day of year used in programming?
Many programming languages have built-in functions to calculate day of year. In Python, use datetime.timetuple().tm_yday; in JavaScript, create a date object and calculate the difference from January 1; in SQL, functions like DAYOFYEAR() are commonly available.
Why would I need to know the day of year?
Day numbers are essential for comparing dates across years, calculating day intervals, creating compact date representations, tracking annual events, and analyzing seasonal patterns in data. Scientists often use them for consistent time measurement.
How accurate is this calculator for historical dates?
Our tool uses the Gregorian calendar system and is accurate for dates after October 15, 1582 (when the calendar was introduced). For earlier dates, consult specialized historical calendar converters as the Julian calendar was previously used.
Can I calculate week numbers with this tool?
While we focus on day numbers, week numbers follow different systems (ISO week dates start with Monday and week 1 contains January 4th). Many programming languages have separate functions for week number calculations.