ISOWEEKNUM takes just one argument,date, which must be a valid Excel date.
The ISOWEEKNUM function increments on Mondays, and starts week 1 on the first week that contains a Thursday.
The WEEKNUM function can also be configured to output an ISO week number, by setting the return_typeargumentto 21.
The WEEKNUM function starts counting on the week that contains January 1.
By default, weeks begin on Sunday, but this can be changed.
