Returns the ISO week number of a given date.
- returns the international standard ISO8601 week number, where week 1 is the week that contains January 4th.
- date is the date as a date-time serial number or as text.
- mode specifies which day is considered the first day of the week: 1 for Sunday, 2 for Monday
- To mimic the non-ISO behaviour of the Excel WEEKNUM function, use WEEKNUM_ADD.
- returns 1. 7Jan10 is a Thursday, and 4Jan10 the Monday of that week.
- WEEKNUM is called ISOWEEKNUM in the forthcoming ODFF standard.