ISOWEEKNUM in Collabora Office: Find ISO Week Numbers at Year Boundaries
Use Collabora ISOWEEKNUM with DATE inputs and keep the ISO week-year separate from the calendar year.
Verification status: See the evidence and test scope described in this guide. How formula examples are checked
Quick answer: =ISOWEEKNUM(DATE(2021;1;1)) has independently expected result 53. That date belongs to the final ISO week of 2020, even though its calendar year is 2021.
Syntax and the ISO week rule
=ISOWEEKNUM(Number)
Number is a numeric date serial or a reference to one. The dedicated Collabora documentation specifies Monday as the first day of the week, with week one containing January 4. Equivalently, the week is assigned to the year containing most of its days.
The result is a week number, generally 1 through 52 or 53. It does not include the ISO week-year. Near New Year, using YEAR(date) alongside the week number can therefore create a misleading label.
Use DATE(year;month;day) in formulas rather than an unquoted date-looking expression containing slashes or hyphens. Those punctuation marks can be parsed as division or subtraction instead of a date.
Example 1: reproduce the documented boundary examples
=ISOWEEKNUM(DATE(1995;1;1)) has documented result 52. The next day, Monday January 2, starts ISO week one of 1995.
=ISOWEEKNUM(DATE(1999;1;1)) has documented result 53. Monday January 4 starts ISO week one of 1999. These examples show why January 1 is not necessarily in week one.
The output is independent of how a valid date is displayed in the cell. Showing January 1 as 01/01 or in a long month-name format does not change the underlying week calculation.
Example 2: build a consecutive year-end table
Put =DATE(2020;12;28) in A2 and =A2+1 in A3, then fill A3 through A9. This creates dates December 28, 2020 through January 4, 2021.
Enter =ISOWEEKNUM(A2) in B2 and fill down. Independent week numbers are 53, 53, 53, 53, 53, 53, 53 and 1. The first seven dates belong to ISO week-year 2020; the last begins week-year 2021.
This fixture crosses both a calendar-year change and a Monday week change. It is more informative than checking a single midyear date where most week conventions agree.
Advanced application: calculate the ISO week-year
For date-only A2, use =YEAR(A2-WEEKDAY(A2;2)+4) in C2 and fill down. WEEKDAY with mode 2 numbers Monday as one, so the expression moves to the Thursday of the same ISO week before reading its year.
For the eight-date dataset, independent years are 2020 for the first seven rows and 2021 for the last. Pair C with B rather than using YEAR(A2) directly to identify the ISO week.
The formula explicitly depends on Monday-based WEEKDAY mode 2. A different weekday mode changes the offset and cannot be substituted without adjusting the construction.
Advanced application: check a December date in the next week-year
December 31, 2018 is a Monday and has independent ISO week number 1 in week-year 2019. January 1, 2019 belongs to that same ISO week.
Grouping only by week number would also merge unrelated week-one records from different years. Keep a two-field key consisting of ISO week-year and week number, or construct a clearly labeled composite after calculating both correctly.
Do not assume all years have 52 ISO weeks. The 2020 boundary fixture demonstrates a valid week 53. A fixed 52-week allocation needs a separate business-calendar rule rather than a silent change to the ISO result.
Errors and troubleshooting
The official help warns about ambiguous date strings and conversion settings. These examples avoid that dependency by constructing numeric dates with DATE. They do not require changing application settings to suppress text-conversion errors.
A bare number is interpreted as a date serial, not as a year or an already calculated week. The workbook’s date base determines its calendar meaning. Date-time fractions, historical serial boundaries, text parsing and imported date systems are not newly native-tested.
The help identifies this function with OpenFormula 1.2. That standards statement is separate from a documented first Collabora release; no introduction date is inferred from it.
Availability and verification
The exact function is documented in Collabora Office 26.04 desktop help. This function’s cited help snapshot was retrieved on 2026-10-15 and hash-verified on October 15, 2026. Desktop documentation does not establish Online or mobile support. First introduction and last functioning release are unknown in this evidence.
No native Collabora Office execution occurred. Worked results are independent logical, calendar, structural or mathematical expectations. Formulas use English names and semicolon argument separators.