With the date January 5, 2016 in B5, WEEKNUM returns 2 as the the week number.
Note: The date must be in a form that Excel recognizes as a valid date.
How this formula works:
The WEEKNUM function takes two arguments, a date, and, optionally, an argument called return_type, which controls the scheme used to calculate the week number.
By default, the WEEKNUM function uses a scheme where week 1 begins on January 1, and week 2 begins on the next Sunday (when the return_type argument is omitted, or supplied as 1). With a return_type of 2, week 1 begins on January 1, and week 2 begins on the next Monday. See the WEEKNUM page for more information.
ISO week number
With ISO week numbers, week 1 starts on the Monday of the first week in a year with a Thursday. This means that the first day of the year for ISO weeks is always a Monday in the period between Jan 29 and Jan 4.
Starting with Excel 2010 for Windows and Excel 2011 for Mac, you can generate an ISO week number using 21 as the return_type:
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The Excel WEEKNUM function takes a date and returns a week number (1-54) that corresponds to the week of year. The WEEKNUM function starts counting with the week that contains January 1. By default, weeks begin on Sunday.
The Excel ISOWEEKNUM function takes a date and returns a week number (1-54) that follows ISO standards, where weeks begin on Monday and week number 1 is assigned to the first week in a year that contains a Thursday.
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