title: Excel weeknum function returns wrong week
body: <p>Question is as in title. </p>
  
  <p>I have a cell, "D4", with the date "09/07/2016" in it. Adjacent cell has formula "=weeknum(D4,1)".  The output of this function is "28".  But on a Sunday-Saturday basis, Saturday the 9th of July wasn't in week 28 - it was in week 27.  </p>
  
  <p>I thought this might be something to do with Saturday/Sunday and when the week starts and finishes, etc, so I tried multiple different dates from last week - Monday the 4th, Tuesday the 5th, Wednesday the 6th, etc.  In each case, "weeknum" returns a value of "28".  </p>
  
  <p>I only noticed the problem because I have a macro which uses the value of the cell with the week number to look for a spreadsheet saved by our accounts team on a weekly basis.  As they have - correctly - saved the spreadsheet as "week 27", it didn't work.  I initially assumed that the accounts team were wrong, but I checked online and they are correct. </p>
  
  <p>How could this happen?  Surely Excel can't be wrong and I must have made a mistake of some sort? </p>
  
  <p>I know that there is an issue with Excel not following ISO standards for when weeks 53 and 1 begin and end, but I don't see how that could affect a mid-year week.  </p>
  
  <p>It did occur to me that the issue might be to do with UK versus US date formatting.  But, of course, the 7th of June wasn't in week 28 either. </p>