The day number of any date, days remaining, the share of the year gone, and the reverse lookup from a day number back to a date.
| Weekday | – |
| ISO 8601 ordinal date | – |
| Quarter | – |
| Day of the month | – |
| Julian Day Number (astronomical) | – |
| Month | Days | 1st is day | Last is day |
|---|
The Day of the Year Calculator converts a calendar date into its ordinal day number, from 1 on January 1 to 365 or 366 on December 31, and shows how many days remain, what share of the year has passed, the ISO week, the quarter, and the ISO 8601 ordinal date such as 2026-256. Switch to the reverse tab to enter a year and a day number and get the calendar date and weekday back, which is how Julian-style dates on food packaging, military paperwork, and some log files are written. A reference table lists the day number on the first of every month for the chosen year so you can convert by hand.
The date is parsed as a local calendar date at midnight and the day number is Math.round((date - January 1) / 86,400,000) + 1, which is immune to the 23- and 25-hour days around daylight-saving changes. Days in the year are 366 when the year is divisible by 4 but not by 100, or divisible by 400, otherwise 365. Days remaining are days in year minus day number, and the percentage of the year gone is day number divided by days in year. The reverse conversion builds January 1 of the year and adds day number minus one days, then rejects numbers above the year's length. The week shown is the ISO 8601 week, where weeks start on Monday and week 1 contains the year's first Thursday.
Manufacturers often stamp a three-digit day of the year, sometimes preceded by a one- or two-digit year, so 6256 or 26256 means day 256 of 2026, which is September 13. Despite the name this has nothing to do with the astronomical Julian Day Number. Use the reverse tab here: enter the year and the three-digit day number to get the packing date.
Only in common years. January has 31 days and February 28, so March 1 is 31 + 28 + 1 = day 60. In a leap year February has 29 days and March 1 becomes day 61. Every date from February 29 onward is one day later in a leap year than in a common year, which is why December 31 is day 366 in 2024 but day 365 in 2026.
This page counts whole days: on day 256 of a 365-day year it reports 256 / 365 = 70.1% complete, treating the current day as finished. A live progress tool measures the exact moment down to the second, so at noon on the same day it would show a value about 0.14 percentage points lower. Both are correct for what they measure.
As the four-digit year, a hyphen, and the three-digit day number: 2026-256 for September 13, 2026. The basic format without the hyphen is 2026256. It sorts correctly as text, which is why it appears in some file names and telemetry systems, and it avoids the month-first versus day-first ambiguity of other formats.
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