run
ISO Week Number
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Converts between calendar dates and ISO 8601 week numbers, in either direction, and reports how many weeks an ISO year has.
Use this whenever a date needs to be expressed as a week number, or a week number needs to be turned back into a date — sprint planning, report periods, "week 37" style scheduling, or reconciling two systems that disagree about what week it is.
DO NOT compute this yourself with day-of-year divided by 7. ISO 8601 week 1 is defined as the week (Monday-Sunday) containing the year's first Thursday, not the week containing January 1st. This has two consequences that are easy to get wrong from memory: late-December dates can belong to ISO week 1 of the NEXT calendar year, and early-January dates can belong to ISO week 52 or 53 of the PREVIOUS calendar year. A year has 53 ISO weeks (not the usual 52) exactly when January 1 is a Thursday, or the year is a leap year and January 1 is a Wednesday — a rule nobody carries around, and getting it wrong produces a plausible-looking but incorrect week number with no visible sign of the error.
Input is a single string, and the shape of the string picks the operation:
"2027-01-01" a calendar date (YYYY-MM-DD) -> returns its ISO year, week, and weekday
"2026-W53" an ISO week, no weekday given -> returns the Monday of that week
"2026-W53-5" an ISO week and weekday (1-7, Mon-Sun) -> returns that exact date
"2026" a bare 4-digit year -> returns whether it has 52 or 53 ISO weeks
Refuses rather than guesses: slash-separated dates like "03/04/2025" are rejected as ambiguous (month/day vs day/month), two-digit years are rejected as a guess, invalid calendar dates (Feb 30) are rejected, and a week number beyond what that ISO year actually has (e.g. week 53 of a 52-week year) is rejected and told how many weeks that year has.
Input schema
| Property | Type | Required | Description |
|---|---|---|---|
| input | string | yes | A calendar date "YYYY-MM-DD" (e.g. "2027-01-01"), an ISO week "YYYY-Www" or "YYYY-Www-D" where D is 1-7 for Monday-Sunday (e.g. "2026-W53-5"), or a bare 4-digit year (e.g. "2026") to ask how many ISO weeks it has. |
Raw JSON schema
{
"type": "object",
"$schema": "https://json-schema.org/draft/2020-12/schema",
"properties": {
"input": {
"type": "string",
"description": "A calendar date \"YYYY-MM-DD\" (e.g. \"2027-01-01\"), an ISO week \"YYYY-Www\" or \"YYYY-Www-D\" where D is 1-7 for Monday-Sunday (e.g. \"2026-W53-5\"), or a bare 4-digit year (e.g. \"2026\") to ask how many ISO weeks it has."
}
},
"required": [
"input"
]
}