PlanMaker WEEKNUM Function: Sunday, Monday and ISO Weeks

Week numbers are useful for shipments, timesheets, seasonal dashboards and recurring reports. They are not universally defined: a Sunday-based calendar, a Monday-based calendar that always starts with the January 1 week, and the ISO 8601 week-year system can produce different week numbers for the same date. If two departments combine reports without specifying their chosen convention, records near the new year may be grouped differently.

WEEKNUM returns a number identifying a calendar week; it does not return a complete date, weekday name, or unique year–week key. PlanMaker’s default is the week-start convention used commonly in the United States. Official WEEKNUM documentation.

=WEEKNUM(Date [, WeekStartsOn])

Brackets denote an optional argument in SoftMaker’s reference; do not type the square brackets. The single comma shown in the English documentation separates arguments. Formula separators and date display/input formats may differ across locales; use your installation’s function insertion UI to confirm any localized separators. For unambiguous portable practice formulas, use numeric DATE(year,month,day) inputs rather than typing ambiguous text like 01/05/2020. The vendor documents numeric date storage and regional date-entry behaviour in Entering data in cells.

Which WeekStartsOn mode should you choose?

This complete enumeration follows SoftMaker’s function reference; 2 and 11 specify the same Monday-start rule. Changing the first day from Sunday to Monday is not the same as enabling ISO weeks. Only the special value 21 selects the ISO week-1 rule.

Open a blank sheet. Put headers in A1:E1: Date, Sunday week, Monday week, ISO week (mode 21), ISOWEEKNUM. In A2:A11 enter the DATE(...) formulas shown below, one per row. Display column A as a Date. In B2 enter =WEEKNUM(A2,1), C2 enter =WEEKNUM(A2,2), D2 enter =WEEKNUM(A2,21) and E2 enter =ISOWEEKNUM(A2). Fill B2:E2 down to row 11. A genuine date should be stored as a numeric date value; a date-looking text string is not a reliable substitute. The final ISO-year column below is context calculated outside PlanMaker, not an extra formula entered on the sheet.

Independent expected-output table (Python calendar arithmetic and datetime.date.isocalendar, not observed software output):

A week number alone is insufficient to identify a reporting period: 2019-12-30 has ISO week 1 of 2020, and 2024-12-30 has ISO week 1 of 2025. Do not pair ISO week numbers with the ordinary calendar year without checking the ISO year separately.

1. In A2 on a fresh worksheet enter =DATE(2020,1,5) and format A2 as Date.
2. In B2 enter =WEEKNUM(A2,1). Independently expected 2: Sunday January 5 begins the second Sunday-based week of 2020.
3. In C2 enter =WEEKNUM(A2,2). Independently expected 1: Monday December 30 began the week containing January 1, and the next Monday does not arrive until January 6.
4. In D2 enter =WEEKNUM(A2,21). Independently expected 1: ISO 2020 week 1 spans December 30, 2019–January 5, 2020.
5. In E2 enter =ISOWEEKNUM(A2). Independently expected 1, equivalent to mode 21 for documented ISO behaviour.

SoftMaker itself supplies examples for January 5, 2020 with modes 1 and 2 in its 2026 WEEKNUM page. The source uses a quoted U.S.-style date string; the worksheet above instead constructs an unambiguous date value.

Suppose A2:A11 are order dates. To group shipments by Monday-start non-ISO weeks, use =WEEKNUM(A2,2) in a helper column, then aggregate by both the calendar year and the calculated week. If your organization requires ISO reporting, use =WEEKNUM(A2,21) and group by ISO week-year, not YEAR(A2). For December 30, 2024, YEAR(A2) is independently expected 2024, yet the ISO year is 2025: joining them as 2024-W01 is a labeling error.

PlanMaker documents a separate, non-Excel-compatible function, ISOWEEK(Date [, ReturnYear]): =ISOWEEK(A2,1) asks for the ISO year, and =ISOWEEK(A2) for the ISO week. The expected ISO year for December 30, 2024 is 2025, but this formula has not been executed here. SoftMaker ISOWEEK reference.

For any date A2, place =WEEKNUM(A2) and =WEEKNUM(A2,11) in adjacent cells; the first uses Sunday and the second Monday. Compare against =WEEKNUM(A2,21) to reveal any difference caused by the ISO week-1 rule, especially near New Year’s Day. Example: for 2021-01-01, independently expected values are 1, 1 and 53, respectively. The ISO result belongs to ISO year 2020.

Keep the week convention in a documented report column or a visible heading; otherwise future editors may interpret the same result as another standard. If a workbook moves into Excel, documented numeric mode values generally align, but date serials, locale parsing and untested edge cases still need checking.

WEEKDAY(A2,2) answers which day of the week a date falls on (Monday=1 through Sunday=7), not which numbered week of the year it belongs to. For January 5, 2020, the independently expected WEEKDAY(A2,2) result is 7 (Sunday); WEEKNUM(A2,2) is expected 1. These functions are not interchangeable. SoftMaker WEEKDAY reference.

The specific error codes and invalid-input coercion for PlanMaker are not established by these documentation checks; test invalid dates, unsupported mode values, empty cells and nonnumeric strings natively before claiming exact responses. The vendor’s date-entry documentation documents January 1, 1900–December 31, 2500 as a date-entry range.

The WEEKNUM syntax and documented mode list were checked in official English manuals for PlanMaker 2021, 2024 and 2026, all on 2026-10-12. The presence of documentation does not prove the earliest version supporting the function, nor behavior in every platform/edition/localization: native Windows/macOS/Linux tests were not conducted. The formula examples use the English names and comma notation of those manuals.

Microsoft Excel WEEKNUM documentation describes WEEKNUM(serial_number,[return_type]) with modes 1, 2, 11–17 and 21; SoftMaker specifically warns that Excel modes 11–17/21 require Excel 2010 or later. Excel uses a different argument name (return_type) but shares the documented week conventions; this does not imply that locale/date-string parsing, error codes, or export roundtrips are identical. PlanMaker 2026.

• SoftMaker PlanMaker 2026: WEEKNUM — exact syntax, default, all mode values, example, Excel-2010 mode note.
• SoftMaker PlanMaker 2024: WEEKNUM and PlanMaker 2021: WEEKNUM — compared function documentation and version presence.
• SoftMaker PlanMaker 2026: ISOWEEKNUM and ISOWEEK — ISO vs calendar conventions, special ISO-year argument for ISOWEEK.
• SoftMaker PlanMaker 2026: WEEKDAY — weekday numbering differences.
• SoftMaker: Entering data in cells — dates as numbers, locale considerations, input range and formula prefix.
• Microsoft Support: Excel WEEKNUM — Excel syntax and mode semantics, not evidence of PlanMaker native behavior.

Claim/validation classification: Official syntax/version/defaults and reference examples = documentation check; 10-date table, ISO-year rollover and calculations = independent Gregorian arithmetic; PlanMaker formula results = expected, not observed; unsupported-input error names, runtime/platform parity = not verified.