Week number formats and week codes

Cheat sheet: every format

All rows below describe the current week:

ISO 8601 notation

The standard form is YYYY-Www: four-digit ISO week-year, a literal W, and a two-digit week (W01–W53). Add -D for a specific day: 2026-W29-1 is this week's Monday. Two details people miss:

• The year part is the week-year, not the calendar year — January 1, 2027 is 2026-W53-5, not 2027-….
• The week is always zero-padded. 2026-W7 is not valid ISO notation and breaks sorting; write 2026-W07.

YYWW date codes — manufacturing and electronics

Flip a chip, a battery, or a tyre over and you'll often find a four-digit code like 2629: two-digit year, two-digit week of manufacture. This YYWW "date code" is the de facto standard for electronics components, appears in serial numbers, PCB silkscreens, food and pharma lot numbers, and automotive parts (DOT tyre codes use the reverse, WWYY).

Reading one is easy once you know the convention: 2629 = year 2026, week 29 — decode the week to real dates with the calculator or the year charts. Two warnings: some manufacturers use WWYY order (a code like 0426 could be week 4 of 2026 or week 26 of 2004), and the two-digit year repeats every century — context has to supply the rest.

CW, KW and WW

CW ("calendar week") is the standard abbreviation in European business English: "delivery in CW32". It is a direct translation of the German KW (Kalenderwoche), and in both cases the numbering is ISO 8601. If a German colleague says "KW 29", that is exactly this week.

WW ("work week") dominates in engineering-driven US companies — semiconductors especially; schedules read "tape-out WW37". Most firms define WW identically to ISO, but a minority number from January 1 with Sunday-start weeks, which drifts one off ISO in some years. When a supplier quotes a WW date, it is worth one email to confirm which system they mean — week start conventions explains how the two diverge.

Week codes in file names and tickets

Week codes shine as prefixes: 2026-W29-status-report.xlsx sorts chronologically forever, groups naturally by year, and never collides with month naming. Three rules make it work:

• Always four-digit year + zero-padded week (2026-W29, never 26-w29) so lexicographic order equals date order.
• Use the ISO week-year for late-December files, or a December 30 file named …-W01 will confuse people (it's correct!) — add the date inside the document if the audience is mixed.
• Keep the W. A bare 2026-29 reads as a month.

Parsing pitfalls

• Week-year vs calendar year — the classic bug: formatting a date with the week-based year pattern (YYYY in Java/Swift date formats, vs yyyy) silently shifts New Year dates by a year. If your app shows "January 1, 2026" as 2025 or 2027 anywhere, this is why.
• W53 validation — 2027-W53 does not exist; week 53 only exists in 53-week years. Validate against the year's actual week count, not a constant 53.
• Locale week numbers — spreadsheet WEEKNUM defaults and OS locales may use Sunday-start numbering. If two systems disagree by one, compare their week-start settings first.