The ISOWEEKNUM function accepts a date and returns the ISO week number (1 through 54) of that date. The date must be in a valid format, so you have to use the DATE function, wrap it in quotes, or use a cell reference to a cell containing a date.
An ISO week starts on Monday. The first week of the year is the first week containing a Thursday in that year.
If your days, months, and years are in different cells, use the DATE function to combine them.
date– The date for which you want the ISO week number. This is required.
DATE – Takes separate year, month, and day values and returns them as a date
DATEVALUE – Returns the serial value of a date
WEEKNUM – Returns the number of a given week
#NUM – The inputs are numbers but are not valid dates for the date input or the type input is out of range. This could happen if you use a negative number for the date.
#VALUE! – The inputs are something that doesn’t convert to a number such as “The other day” or “Yester-yester-day”.
Example 1 – Plain and Simple
Example 2 – ISOWEEKNUM with DATE function
Live examples in Sheets
Go to this spreadsheet for the examples of the ISOWEEKNUM function shown above that you can study and use anywhere you would like.