[EWS] Adding due date to flagged message

I am using EWS (direct XML, not the managed API) to create "flagged" emails in the inbox.  I do this by setting PR_FLAG_STATUS to the appropriate value.  This works, however, when viewed in Office 365, there is no due date set.  I have tried setting the due date using the following extended field URI code:

<t:ExtendedProperty>
<t:ExtendedFieldURI DistinguishedPropertySetId="Task" PropertyTag="0x8105" PropertyType="SystemTime"/>
<t:Value>2014-02-02T00:00:00Z</t:Value>
</t:ExtendedProperty>

This however results in the EWS failure "The extended property attribute combination is invalid".  Is there anything I can do to set the due date field in a flagged message?