The tasks:DateCompleted element is an optional child element of the Flag element (section 126.96.36.199) that identifies the date on which a flagged item was completed. It is defined as an element in the Tasks namespace.
The tasks:DateCompleted element is required to mark a flagged item as complete. If a message includes a value for the tasks:DateCompleted element, the CompleteTime element (section 188.8.131.52) is also required. The value of this element is a dateTime data type, as specified in [MS-ASDTYPE] section 2.3.
A maximum of one tasks:DateCompleted child element is allowed per Flag element.