What day of the year is it?
The day of year (or ordinal day) numbers each day from 1 on 1 January to 365 — or 366 in a leap year — on 31 December. It is widely used in manufacturing date codes, agriculture, logistics and IT systems, where it is sometimes loosely called the “Julian day”. This calculator returns the day number, days remaining and ISO week for any date.
How it works
For the date you pick, the tool counts the whole days from 1 January of that year and adds 1, so 1 January is day 1. It checks whether the year is a leap year (divisible by 4, but century years must be divisible by 400) to set the total at 365 or 366 days, then:
- Days remaining = total days − day of year
- ISO 8601 week number = the week containing the year’s first Thursday is week 1, with weeks running Monday to Sunday
All arithmetic uses UTC so the result never shifts across time zones.
Worked example
For 10 April 2026 (a common year):
- Day of year: 31 (Jan) + 28 (Feb) + 31 (Mar) + 10 = day 100 of 365
- Days remaining: 365 − 100 = 265
- ISO week: week 15
| Date | Day of year | Days remaining | ISO week |
|---|---|---|---|
| 2026-01-01 | 1 of 365 | 364 | 1 |
| 2026-04-10 | 100 of 365 | 265 | 15 |
| 2024-12-31 | 366 of 366 | 0 | 1 (next year) |
Where the ordinal day appears in real life
Manufacturing lot codes. Many food, pharmaceutical, and electronics manufacturers encode the production date as a 3-digit ordinal day followed by a 2-digit year — for example, 10026 means day 100 of 2026 (April 10). Reading these codes quickly tells you the manufacturing date and how fresh or old a batch is.
Agriculture and horticulture. Growing degree day models and crop schedules are referenced by ordinal day. A planting guide that says “sow at day 90” means March 31 in a common year or March 30 in a leap year. Knowing the ordinal day lets you cross-reference schedules that do not use calendar months.
Aviation and logistics. Aircraft maintenance intervals and cargo tracking systems often record dates as ordinal day numbers in their internal logs, particularly in legacy systems where the Julian date was space-efficient.
Programming. Python’s datetime.timetuple().tm_yday, .NET’s DayOfYear, and SQL’s DAYOFYEAR() function all return the ordinal day. When debugging a date-related bug involving these values, this calculator lets you verify what a given number actually corresponds to.
Deadline and project tracking. Knowing you are on day 100 of 365 (27.4% through the year) or that 265 days remain gives a quick sense of where you stand on an annual goal or budget.
ISO week number edge cases
The ISO week number occasionally surprises people at year boundaries. December 31 in some years falls in ISO week 1 of the following year (as in 2024-12-31 above), because ISO week 1 is the week containing January’s first Thursday. Conversely, January 1 can fall in week 52 or 53 of the previous year. The calculator handles these edge cases correctly.
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