WEEKNUM in Zoho Sheet: Sunday, Monday and ISO Weeks

=WEEKNUM(date; [mode])

Zoho’s official WEEKNUM function reference lists all these modes:

System 1 does not require a full week in January; the week containing January 1 is always numbered 1. The ISO system instead assigns week 1 to the week containing the first Thursday, equivalently the week containing January 4. ISO weeks begin Monday, and a few dates near New Year’s belong to an ISO week-year that differs from the calendar year.

Create a worksheet named DatePeriods with the following fields in A1:E9. For WEEKNUM, reference only D: Report date (rather than B:C, which the companion duration tutorials use). Type D as genuine spreadsheet dates or use DATE(year;month;day) so your locale does not reinterpret an ISO-looking string.

A CSV with the same values accompanies this review batch; it has not been imported into Zoho. Formula examples below use semicolons as documented by Zoho’s online reference.

1. In F1, type Sunday week, in G1, Monday week, and in H1, ISO week.
2. In F2, enter =WEEKNUM(D2); this is mode 1.
3. In G2, enter =WEEKNUM(D2;2).
4. In H2, enter =WEEKNUM(D2;21).
5. Copy F2:H2 through row 9. Set F:H to Number format with zero decimals.
6. Compare with this independently computed calendar-model table; these are not native Zoho Sheet executions:

Understanding the New Year trap

December 30, 2024 is a Monday. Under ISO rules it starts week 1 of 2025, while the calendar year remains 2024. Reporting with only the week number would mix this row with other years’ week 1. Store both ISO week-year and week number in reports; do not use YEAR(D7) as the ISO year just because you used ISO mode 21 for the week number.

Why the Sunday and Monday modes differ on October 11, 2026

October 11, 2026 is a Sunday. In system 1, a Sunday starts a new week in mode 1, giving independently week 42. In mode 2 it is the last day of the Monday-starting week, giving week 41. This choice affects weekly aggregates even when the year boundary is nowhere near the report date.

If D2 contains January 1, 2021, enter:

=YEAR(D2-WEEKDAY(D2;2)+4)&"-W"&WEEKNUM(D2;21)

The independent result is 2020-W53. WEEKDAY(date;2) returns Monday as 1 under Zoho’s documented mode 2; subtracting that weekday offset and adding 4 moves the date to the Thursday of its ISO week, so YEAR supplies the correct ISO year. This is a useful formula-level construction, not a function executed natively in Zoho during drafting.

For D7, December 30, 2024, the same expression independently yields 2025-W1. This simplified key does not zero-pad week numbers; for a fixed-width code such as 2025-W01, add a carefully documented text-formatting step and verify the TEXT function separately.

For a quick workflow flag showing that two weekly calendars disagree, =IF(WEEKNUM(D5;1)<>WEEKNUM(D5;21);"Different";"Same") independently evaluates to Different for the October 11, 2026 example. This is useful when reconciling supplier reports with different definitions.

• Week number without year is ambiguous: Week 1 occurs every calendar/ISO year. A proper reporting key needs a year plus a week value chosen under the same calendar rules.
• Mode 21 is ISO: It follows the first-Thursday rule and may label dates at year-end as week 1 of the next ISO year. A YEAR(date) calculation alone may not provide that week-year.
• mode must be supported: Zoho explicitly lists 1, 2, 11–17, and 21. An invented mode is not proven to work. Microsoft Excel documents a #NUM! response for an unsupported mode; Zoho’s own page lists generic error categories without guaranteeing that precise outcome.
• #VALUE!: Zoho’s general help explains invalid argument types. Check whether D2 is an actual date value, especially after importing a CSV.
• #NAME!: Check spelling WEEKNUM, parentheses, and the separator appropriate to the running environment.
• Missing report dates: Do not silently treat blanks as meaningful dates. Add an explicit data-quality check or filter before generating weekly summaries.
• Disagreement with WEEKNUM_ADD: Zoho separately lists a WEEKNUM_ADD function described as non-ISO, yet that reference also includes mode 21 in its table. Its precise distinction from WEEKNUM is not established by those descriptions alone and needs native test cases; do not assert that it is an exact alias.
• Week 53: Both system 1 and ISO can produce a 53rd week in some years, but not necessarily for the same dates or week-year.

Microsoft Excel documents equivalent mode numbers and System 1/System 2 rules in its WEEKNUM reference, with commas in its English function examples. Zoho’s online help uses semicolons. A workbook coming from Excel should be checked for actual formula parsing and boundary results after import; equivalent-looking function names do not prove identical runtime outcomes across every platform. Google Sheets has its own weekly functions, but no native Google Sheets formula was executed or relied upon here.

Zoho’s Windows/macOS desktop beta announcement says 400+ functions were supported while some web features were still being added. This does not establish a guaranteed first-supported WEEKNUM version or complete uniform availability across browser, desktop, mobile, accounts and old files. The page is researched against the presently available online Zoho reference, not a historical compatibility matrix.

• WEEKDAY — previously prepared review guide for weekday positions; live URL not verified.
• YEAR — previously prepared review guide for calendar years, which are not always ISO week-years.
• WEEKNUM_ADD — officially listed separate function; research candidate, not presumed identical to WEEKNUM.
• DATE — previously prepared review guide for constructing unambiguous dates.

Internal link targets should only be inserted at publication time after confirming the actual approved pages are live.

• Zoho Sheet — function documentation
• Zoho Sheet WEEKNUM_ADD official reference
• Zoho Sheet Windows/macOS desktop beta announcement

Verification status: Official documentation checked; example outcomes were independently reasoned or arithmetically checked where applicable. Not executed in Zoho Sheet.

• No genuine Zoho Sheet screenshots are included; this guide does not use mock application images.