A few reasons why the WEEKNUM formula may not work are, • The cell reference given for the argument is not a valid date. • We have given the date value directly as an argument, and have not enclosed it in double-quotes. In such scenarios, we get an incorrect result. • We get the “#NUM!” error if the date values are out of range or not in the right format. • The result becomes 52 when we delete the dataset or there are empty or blank cells.