This post discusses about the support of “Um-al-Qura calendar” in Office 2010, that varies in different applications of Office 2010, and we’ll go over some of the features that use the Um-al-Qura calendar. Applications Dependent On The Operating System: Support for Um-al-Qura calendar's available in Vista and above. If you set Current Format in Regional and Language Options in Vista to Arabic (Saudi Arabia), and then select Customize –> Date, you’ll notice default calendar's set to Um-al-Qura. These options're Format in Region and Language and Additional Settings –> Date in Windows 7 respectively. For other Arabic formats that don’t have Um-al-Qura as default, we can manually change calendar to Um-al-Qura. In 2010, “Excel, InfoPath, and Visio” app will've support for Um-al-Qura only if operating system is Windows Vista or a later version, with calendar type set to Um-al-Qura as described above.
More info: Um-al-Qura Calendar Support in Office 2010