If you'll try to use a usual procedure(doc.ReplaceItevValue) for changing values of Date/time fields, you'll fail to do it, becouse this procedure is changing the field type to text.
Here is a code that solves this problem:
Suppose you have a field "date" which stores some date values you are about to change...
Code
Code: Sub Initialize
Dim w As New NotesUIWorkspace
Dim dc As NotesDocumentCollection
Dim doc As NotesDocument
Dim Item As NotesItem
Dim nDate As NotesDateTime
Set dc=w.CurrentView.Documents
If dc.Count>0 Then
d=Format(Cdat(Inputbox ("Please input a new date value", "New date")),"dd.mm.yyyy")
Set nDate=New NotesDateTime(d)
Set doc=dc.GetFirstDocument
While Not (doc Is Nothing)
Set item=doc.GetFirstItem("date")
Set item.DateTimeValue=nDate
Call doc.Save(True,True)
Set doc=dc.GetNextDocument(doc)
Wend
End If
End Sub