wrong week number using Weeknum(date,x)

Hello Zalvi, welcome to MrExcel

It sounds like you want ISO week numbers (where week 1 starts on the first Monday on or after 29th December). There's no inbuilt function for those. You can download morefunc add-in and use ISO.WEEKNUM function or use this formula

=INT((A2-WEEKDAY(A2,2)-DATE(YEAR(A2+4-WEEKDAY(A2,2)),1,4))/7)+2

where your date is in A2

Hello barry,

I was pleasantly surprised when I found in Excel 2010 that
function is compatible with the Gregorian Calendar now.

Reading Excel 2010 help about WEEKNUM function:
System 2: The week containing the first Thursday of the year is the first week of the year, and is numbered as 1 week. This system is specified in the ISO 8601, which is commonly known as the European week numbering system.

And this is achieved with a value of return_type = 21 for all countries following the Gregorian Calendar and ISO 8601 standard.

It is a pity that this return_type was not included before in versions prior to Excel 2010.

In the 2010 Beta version I'm trying, do not show the type 21 when I'm writing the WEEKNUM() function, only types 1, 2 and 11 to 17 are showed. Could Microsoft fix it?

@Zalvi if argument has a value of 1 or omitted Week begins on Sunday.

In Excel 2003 and 2007, the WEEKNUM function considers the week containing January 1 to be the first week of the year. However, there is a European standard that defines the first week as the one with the majority of days (four or more) falling in the new year. This means that for years in which there are three days or less in the first week of January, the WEEKNUM function returns week numbers that are incorrect according to the European standard.

In Excel 2010, use one of these formulas:
(A1 is a date format cell)

MS Excel remember us that dates should be entered by using the DATE function, or as results of other formulas or functions. For example, use DATE(2010,5,23) for the 23rd day of May, 2010. Problems can occur if dates are entered as text.