The Excel Isoweeknum function returns the ISO week number of a supplied date. This function was only introduced in Excel 2013 and so is not available in earlier versions of Excel.
Syntax: ISOWEEKNUM( date )
It is recommended that the supplied date argument is input as either a reference to a cell containing a date or a date returned from a formula. If you attempt to input the date as text, there is a chance that Excel may misinterpret this due to different date systems, or date interpretation settings.
ISOWEEKNUM Function Examples
ISOWEEKNUM Function Errors
#NUM! – Occurs if the supplied date argument is a numeric value, but is not recognised as a valid date.
#VALUE! – Occurs if the supplied date argument is a text value that can not be interpreted as a date.