What Is a Day of Year Calculator?
A day of year calculator determines the ordinal day number (1-365/366) for any given date within its year. It also shows the remaining days until year end, the ISO week number, the quarter, and whether the year is a leap year. This information is useful for planning, scheduling, ordinal date systems, and data analysis.
How to Use This Day of Year Calculator
- Select a date using the date picker.
- Click Calculate to see the day number, days remaining, week number, and more.
Key Concepts
The day of year (DOY) counts from January 1 as day 1. A regular year has 365 days, while a leap year has 366. The ISO week number follows the ISO 8601 standard where weeks start on Monday and the first week of the year contains January 4. Understanding these ordinal dates is important for agriculture, astronomy, and systems that use Julian day numbering.
Frequently Asked Questions
What is an ISO week number?
The ISO week number follows the ISO 8601 standard. Weeks start on Monday, and the first week of the year is the one containing the first Thursday of January (equivalently, the week containing January 4). A year has either 52 or 53 ISO weeks.
How is a leap year determined?
A year is a leap year if it is divisible by 4, except for century years which must also be divisible by 400. So 2000 and 2024 are leap years, but 1900 and 2100 are not. Leap years add February 29, making the year 366 days long.