Day of the Year Calculator
Enter any date to instantly find its day-of-year number (also called the ordinal date or Julian day number), how many days remain in the year, the ISO week number, the quarter, and what percentage of the year has passed. Switch to Reverse mode to convert any day number back to a full calendar date. Leap years are handled automatically.
What is the day of the year?
The day of the year, also called the ordinal date or Julian day number, is a count of how far into a calendar year a date sits. January 1 is day 1, January 2 is day 2, and so on through to December 31, which is day 365 in a common year and day 366 in a leap year. Ordinal dates are widely used in astronomy, meteorology, the military, aviation, and computer systems because a single integer is easier to store, compare, and subtract than a full month-day-year triplet.
How to calculate the day of the year
To find the day number for any date, add up the number of days in every month before the target month, then add the day of the month. For example, June 19 in a common year: January (31) + February (28) + March (31) + April (30) + May (31) = 151, then add 19 to get day 170. In a leap year, February has 29 days instead of 28, so every date from March 1 onward is one day later than its common-year equivalent. The formula for days remaining is simply the total days in the year (365 or 366) minus the day number.
Reverse lookup: day number back to a calendar date
Converting a day number back to a calendar date is the reverse of the above process. Start with the ordinal and subtract each month total in order until the remainder falls within a month. For example, day 170 in a common year: subtract January (31) to get 139, subtract February (28) to get 111, subtract March (31) to get 80, subtract April (30) to get 50, subtract May (31) to get 19. Since 19 is no more than June's 30 days, the date is June 19. Switch to Reverse mode in this calculator and it performs the same arithmetic instantly.
Leap years and how they affect day numbers
A leap year occurs when the year is divisible by 4, except for century years (divisible by 100), which must also be divisible by 400. So 2000 and 2400 are leap years, but 1900 and 2100 are not. In a leap year, February has 29 days, which shifts every date from March 1 onward one position later compared with a common year. Day 60 is February 29 in a leap year and March 1 otherwise. This calculator detects leap years automatically and adjusts all outputs accordingly.
ISO week numbers and quarters
This calculator also shows the ISO 8601 week number. Under that standard, weeks start on Monday, and week 1 is the week containing the first Thursday of the year (equivalent to the week containing January 4). The result ranges from 1 to 52 in most years and 53 in some years that start or end on certain weekdays. The quarter output uses the standard calendar split: Q1 covers January through March (days 1-90 in common years, 1-91 in leap years), Q2 covers April through June (days 91-181), Q3 covers July through September (days 182-273), and Q4 covers October through December (days 274-365/366).
Notable day numbers of the year
| Day Number | Typical Date | Note |
|---|---|---|
| 1 | January 1 | New Year's Day |
| 32 | February 1 | Start of meteorological winter (Southern Hemisphere) |
| 60 | March 1 (or Feb 29) | Day 60 is Feb 29 in leap years, Mar 1 otherwise |
| 91 | April 1 | Start of Q2 |
| 100 | April 10 / 11 | Day 100 milestone, popular school celebration |
| 172 | June 21 (approx) | June solstice (Northern Hemisphere summer) |
| 183 | July 2 | Exact midpoint of a common year |
| 184 | July 2 / 3 | Midpoint of a leap year |
| 256 | September 13 | Programmers Day (256 = 2^8) |
| 274 | October 1 | Start of Q4 |
| 355 | December 21 (approx) | December solstice (Northern Hemisphere winter) |
| 365 | December 31 | Last day of a common year |
| 366 | December 31 | Last day of a leap year only |
Commonly referenced ordinal dates, valid for both common (365-day) and leap (366-day) years unless noted.
Frequently asked questions
What day of the year is today?
Select today's date in the calculator and it will instantly show the ordinal day number, days remaining, and year progress. In 2026, June 19 is day 170 of 365.
How do I calculate the day of the year by hand?
Add the number of days in every month that has already completed before your target month, then add the day of the month. For dates from March onward, add an extra 1 if the year is a leap year. For example, March 15 in a common year is 31 (Jan) + 28 (Feb) + 15 = day 74; in a leap year it is 31 + 29 + 15 = day 75.
What is day 100 of the year?
Day 100 is April 10 in a common year and April 9 in a leap year. It is a popular milestone in schools and communities for marking roughly the first quarter of the year, and some countries celebrate "Day 100" events.
What is Programmers Day and why is it day 256?
Programmers Day is celebrated on day 256 of the year - September 13 in a common year, September 12 in a leap year. The number 256 was chosen because it equals 2 to the power of 8, which is the number of distinct values that can be stored in a single byte of memory, a number deeply familiar to software developers.
What is the difference between the ordinal date and the Julian Day Number?
The ordinal date (day of the year) simply resets to 1 every January 1. The Julian Day Number (JDN) used in astronomy is a continuous count of days starting from noon on January 1, 4713 BC, and never resets. They are related but not the same - this calculator computes the ordinal date, not the astronomical Julian Day Number.
Can I convert a day number back to a calendar date?
Yes. Switch the Mode selector to "Day number to date", enter the year and the day number (1-365 or 1-366 for leap years), and the calculator shows the corresponding month and day along with the weekday, ISO week, and quarter.
Why does February's length matter for day-of-year calculations?
February is the only month whose length changes between years. In a common year it has 28 days; in a leap year it has 29. This shifts the day number of every subsequent date by exactly one. If you calculate a day number for any date from March through December, you must know whether the year is a leap year to get the correct result.