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WEEKNUM  function returns the number of week of a date, that is, whether a date is the 5th, or 17th week of a year etc. Syntax WEEKNUM(date, [ type ]) type is optional.  If type is 1, it means the week starts on Sunday. The default value of  type  is 1. If type is 2, it means the week starts on Monday.  If  type  is 21, WEEKUM is equivalent to ISOWEEKNUM type  can be other values, which are rarely used. See the official reference if you need to use it. Example

ISOWEEKNUM returns the number of week of a date using ISO standard, that is, whether a date is the 5th, or 17th week of a year etc. Syntax ISOWEEKNUM(date) Example Notes Per ISO standard, a week begins on Monday and ends on Sunday. Under ISO standard, whether a week belongs to a years depending whether its Thursday falls in a year. Therefore, January 1 is not always week 1 of a year, it can be week 53 of the last year. Similarly, December 31 is not always week 52, it can week 53 of the current year, of week 1 of the next year.