WEEKNUM returns the week number of @date according to the given @method.
@method defaults to 1.
For method=1, week starts on Sunday, and days before first Sunday are in week 0.
For method=2, week starts on Monday, and days before first Monday are in week 0.
For method=150, the ISO 8601 week number is returned.
Returns #NUM! if date or method is invalid.
This function is Excel compatible, except that Excel does not support ISO 8601 week numbers.