ISO week number in Excel
Prior to Excel 2013, the only way to calculate the ISO week number in Excel was to use complex formula as shown in various blog posts.
For clarity, the ISO Week number has been developed by the The International Organization for Standardization (ISO). You can find out more about it on Wikipedia.
In Excel 2013 there is now a function that allows you to calculate the week number called ISOWEEKNUM. It is a very simple function and all you need to do it point at the relevant date and it will generate the ISO Week Number.