royaltiger wrote:Formula Description Result

=WEEKNUM(A2) Number of the week in the year that 3/9/2012 occurs,

based on weeks beginning on Sunday (default). Week 10

=WEEKNUM(A2,2) Number of the week in the year that 3/9/2012 occurs,

based on a week beginning on Monday (the second argument, 2). Week 11

Hope this helps

Please note that when you need week numbers for use in Europe, WEEKNUM(A2,2) will give you incorrect results for years when January 1st falls on a Friday, Saturday, or Sunday.

You need to use WEEKNUM(A2,21) instead. Function code 2 does not provide correct ISO week numbers in these years.

I'm sorry I didn't quote the Excel help comprehensively enough. In "System 1" as mentioned in my previous posting, week1 is always the week that contains January 1st. That's what is used in the US. In "System 2", the system used by ISO, the week containing January 1st is week 1 only if January 1st occurs no later than Thursday. Otherwise, it's week 53 of the old year and week 1 is the following week.

WEEKNUM(A2,2) uses Sytem 1 while WEEKNUM(A2,21) is using System 2. Check Excel help if it's still unclear.