In a Microsoft Exchange Server 2010 environment, the week numbers that are displayed in the Outlook Web App calendar do not match the week numbers that are displayed in the Microsoft Outlook calendar. For example, in the English version of Outlook Web App, the first week in 2013 is numbered "2." However, in the English version of Microsoft Outlook 2010, the first week in 2013 is numbered "1."
To display the week number in Outlook Web App, select the Show week numbers in Month View check box in the Calendar settings.
To display the week number in Outlook 2010, select the Show week numbers in the month view and in the Date Navigator check box. This check box is located under the Display options heading in the Calendar tab of the Outlook Options dialog box.
This issue occurs because Outlook Web App uses a different mechanism to calculate week numbers.
To resolve this issue, install the following update rollup:
2746164 Description of Update Rollup 6 for Exchange Server 2010 Service Pack 2
Note After you apply this update rollup, you can run the Get-MailboxCalendarConfiguration cmdlet together with the FirstWeekOfYear parameter to display the first week of the year. Additionally, you can run the Set-MailboxCalendarConfiguration cmdlet together with the FirstWeekOfYear parameter to set the first week of the year based on one of the following values:
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about a similar issue that occurs in an Exchange Server 2007 environment, click the following article number to view the articles in the Microsoft Knowledge Base:
2289607 Week numbers do not match between OWA and Outlook calendars in the English and French versions in an Exchange Server 2007 environment
For more information about the Get-MailboxCalendarConfiguration cmdlet, go to the following Microsoft website: