There are several ways to refer to dates. You can use exact dates like "January 1st, 2008", or you may use less precise descriptions like "in April 2008" or "next year". Usually everybody familiar with the western calendar will know what timeframe you are talking about. That's not the case if you're referring to week numbers, as there are several ways to calculate them.
The US have a fairly simple scheme for calculating the week number: The week which contains January 1st is defined as the first week of the year. The only drawback of this approach is that the last days of a year may fall into the first week of the following year.
Most european countries refer to ISO 8601 to enumerate weeks. The first week is the week that contains at least 4 days of the new year.
This is tricky enough if you have to sync an appointment with people who use a different calendar than you, but it gets worse if you need to calculate the week number in an application:
Excel has an build-in function
=KALENDERWOCHE() to calculate the week number. However this function can only calculate the US variant of a week number and it does not support ISO 8601.
So: Be aware that there may be cultural differences in the functions you are using and keep in mind that a localized function name does not mean that the function itself is localized, too.
Use the following formula, if you need to calculate the ISO 8601 week number in Excel: