ISO Week Date Format — YYYY-Www-D Explained

Updated: May 2026

ISO 8601 defines a week date notation that compresses year, week number, and day of week into a single unambiguous string. Understanding this format avoids confusion when reading machine-generated dates in APIs, logs, and scheduling systems.

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The format at a glance

The ISO 8601 week date format is written as:

YYYY-Www-D

YYYY — the ISO year (four digits). This is the year that owns the week, which may differ from the calendar year for the first or last days of January/December.
W — a literal capital letter W, used as a separator to signal the week component.
ww — the two-digit ISO week number, from 01 to 53, always zero-padded.
D — the day of the week as a single digit: 1 for Monday through 7 for Sunday.

The separating hyphens are optional in the compact form. The extended form 2026-W22-4 and the compact form 2026W224 are both valid ISO 8601. Most software and APIs use the extended hyphenated form for readability.

Example: 2026-W22-4 means Thursday (day 4) of week 22 of ISO year 2026. Converting this to a Gregorian date gives May 28, 2026.

Why ISO year differs from calendar year

The ISO year follows the week, not the calendar. Because weeks must be complete Monday-to-Sunday units, December 31 can belong to ISO week 1 of the following year if December 31 is a Monday, Tuesday, or Wednesday. In that case the ISO year in the date string would be the next calendar year, even though the Gregorian date is still in December.

Similarly, January 1, 2 or 3 can have an ISO year equal to the previous calendar year if those days belong to the last week of that year.

This is precisely why the format uses YYYY rather than simply relying on a calendar year assumption. A date like 2015-W53-6 (Saturday of week 53 of ISO year 2015) corresponds to January 2, 2016 in the Gregorian calendar.

Where this format appears in practice

The ISO week date format is used in several common contexts:

Manufacturing and supply chain — delivery schedules, production plans, and quality reports often reference ISO week numbers directly.
Project management tools — Gantt charts and sprint planning tools in European markets typically display and sort by ISO week.
APIs and data exports — some reporting APIs return date ranges as YYYY-Www aggregation keys rather than individual dates.
Payroll systems — weekly pay periods, especially in Scandinavia and Germany, are indexed by ISO week.
Broadcasting — television scheduling commonly uses ISO weeks to define programming periods.

Converting ISO week date to Gregorian date

To convert YYYY-Www-D to a Gregorian date, the standard algorithm is:

Find January 4 of the given ISO year — it is always in week 1.
Locate the Monday of the week that contains January 4 (that Monday is the start of week 1).
Add (week − 1) × 7 days to reach the Monday of the target week.
Add day − 1 days to reach the specific day within that week.

The Flowfiles ISO week calculator performs this conversion automatically. Enter any ISO year and week number to get the exact Monday-to-Sunday date range in one step.