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This example moves the due date of all selected task items by four weeks. More precisely, we move by 4*7=28 days in order to preserve the original weekday.
Public Sub MoveTaskDueDates()
Dim Exp As Outlook.Explorer
Dim Sel As Outlook.Selection
Dim Task As Outlook.TaskItem
Set Exp = Application.ActiveExplorer
Set Sel = Exp.Selection
If Sel.Count Then
For Each Task In Sel
Task.DueDate = DateAdd("d", 28, Task.DueDate)