Altered docs according to pull request Activiti#84

userguide/src/en/chapters/ch07b-BPMN-Constructs.xml

@@ -2781,8 +2781,9 @@ assertEquals("Ship Order", task.getName());</programlisting>
               <para>
                 There is an activity extension which allows you to specify an expression in your task-definition to set the initial due
                 date of a task when it is created. The expression <emphasis role="bold">should always resolve to a <literal>java.util.Date</literal>, 
-                <literal>java.util.String (ISO8601 formatted)</literal> or <literal>null</literal></emphasis>. 
-                For example, you could use a date that was entered in a previous form in the process or calculated in a previous Service Task.
+                <literal>java.util.String (ISO8601 formatted)</literal>, ISO8601 time-duration (eg. PT50M) or <literal>null</literal></emphasis>. 
+                For example, you could use a date that was entered in a previous form in the process or calculated in a previous Service Task. In case a time-duration
+                is used, the due-date is calculated based on the current date and time, incremented by the given period.
               </para>
                <programlisting>
 &lt;userTask id=&quot;theTask&quot; name=&quot;Important task&quot; <emphasis role="bold">activiti:dueDate=&quot;${dateVariable}&quot;</emphasis>/&gt;</programlisting>