This formula isn't broken because of a syntax error. It's broken because it's trying to solve a date logic problem with text manipulation. The user's core issue, that `26wk53` stubbornly refuses to roll over into `27wk1`, is a classic clash between how humans read weeks and how spreadsheet functions calculate them. The YEAR function clings to the calendar year, while ISO week numbers belong to a different system entirely. When December 2026 contains week 1 of 2027, the formula has no mechanism to reconcile that conflict. It simply outputs the date's calendar year and the week number, producing `26wk53` when it should produce `27wk1`.
The practical takeaway for anyone building date-driven spreadsheets is this: stop forcing text concatenation to do logic it wasn't designed for. The user's second formula, `=YEAR($C$25+3-MOD($C$25-2,7))*100+ISOWEEKNUM($C$25)`, is closer to a correct approach because it shifts the reference point to align with the ISO week's starting Monday. But it still fails because it multiplies by 100 and adds the week number, producing a numeric value like `202653` that must then be parsed back into text. That's fragile. A better method is to use `=YEAR($C$25-WEEKDAY($C$25,2)+4)` to derive the correct ISO year, then combine it with `ISOWEEKNUM($C$25)` in a single, clean formula that treats year and week as a pair, not as separate text strings.
What this means for you is that the tools you rely on, even familiar ones like WEEKNUM and TEXT, have edge cases that only surface at year boundaries. The solution isn't to patch the formula with more nested conditions. It's to understand that ISO weeks exist in a separate logical domain from Gregorian dates. If you manage project timelines, payroll cycles, or inventory schedules that follow ISO week numbering, build your spreadsheets to treat weeks as their own dimension from the start. Hard-code a reference date, calculate the ISO year explicitly, and never let `TEXT($C$1,"YY")` decide the year for you. That single change turns a recurring headache into a one-time setup.