What this calculator does
Some schedules, planners and systems refer to a date as a plain day number counting from the start of the year, such as "day 45" meaning the 45th day of that year. This calculator turns that number back into an actual calendar date, complete with the day of the week it falls on.
The one detail that trips people up is leap years: a day number past 59 shifts by one day in a leap year, because of the extra 29 February. Entering the correct year keeps the conversion accurate either way.
The formula
Start from 1 January of the chosen year and add (day number minus 1) days, since 1 January itself is day 1, not day 0. The result is the calendar date, and the calculator also reports which weekday it falls on and how many days remain in that year.
| Term | Meaning |
|---|---|
| Day number | A count of days from the start of the year, where 1 January is day 1. |
| Leap year | A year with 366 days instead of 365, adding 29 February, which shifts every later day number by one day compared with a non-leap year. |
The inputs explained
| Field | What to enter |
|---|---|
| Day number of the year | The day-of-year number to convert, from 1 up to 365 or 366. |
| Year | The year the day number belongs to, since leap years change which date a given number falls on. |
When to use it
Reading a Julian-style date stamp
Some logs, manufacturing codes and older systems record dates as a year and day number rather than a month and day, and this converts that back into a readable date.
Planning around a fixed day-of-year milestone
A project or report might reference "day 100 of the year" as a checkpoint, and this shows exactly which date that is for the year in question.
Checking a leap year shift
Comparing the same day number across a leap year and a non-leap year shows how the extra day in February moves every later date by one day.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What date is day N of 2026?
A range of day numbers converted to dates for 2026.
| Day number | Date | Day of the week |
|---|---|---|
| 1 | 1 Jan 2026 | Thursday |
| 50 | 19 Feb 2026 | Thursday |
| 100 | 10 Apr 2026 | Friday |
| 150 | 30 May 2026 | Saturday |
| 200 | 19 Jul 2026 | Sunday |
| 250 | 7 Sep 2026 | Monday |
| 300 | 27 Oct 2026 | Tuesday |
| 365 | 31 Dec 2026 | Thursday |
How does a leap year shift the same day number?
Day number 100 held fixed while the year varies between leap and non-leap years.
| Year | Date | Year has |
|---|---|---|
| 2024 | 9 Apr 2024 | 366 days (leap year) |
| 2025 | 10 Apr 2025 | 365 days (not a leap year) |
| 2026 | 10 Apr 2026 | 365 days (not a leap year) |
| 2027 | 10 Apr 2027 | 365 days (not a leap year) |
| 2028 | 9 Apr 2028 | 366 days (leap year) |
Questions
Is 1 January day 0 or day 1?
Day 1. This calculator, and the common convention, treats 1 January as the first day of the year, so day 1 always maps back to 1 January.
What is the highest valid day number?
365 in a non-leap year, or 366 in a leap year. The calculator checks the year entered and will flag a day number outside that range for the year given.
Why does the year matter if I just want a day number?
Because leap years insert an extra day, 29 February, partway through the year. Any day number after that point shifts by one calendar day between a leap year and a non-leap year, so the year has to be known to convert correctly.
Can I use this to find how many days are left in the year?
Yes, the result includes the days remaining in the chosen year after the date the day number falls on.
To count between two calendar dates instead of by day number, see the Days Until a Date Calculator.