ISOWEEKNUM returns the ISO 8601 week number of @date.
Returns #NUM! if date is invalid.
An ISO 8601 week starts on Monday. Weeks are numbered from 1. A week including days from two different years is assigned to the year which includes the most days. This means that Dec 31 could be in week 1 of the following year, and Jan 1 could be in week 52 or 53 of the previous year.