Dim dic As New Dictionary(Of Date, Double)
 
With dic
   .Add(CDate("5.4.2011"), 1000)
   .Add(CDate("7.4.2011"), 1200)
   .Add(CDate("6.4.2011"), 1100)
   .Add(CDate("4.4.2011"), 900)
End With
 
Dim valuesum As Double = _
Aggregate kvp As KeyValuePair(Of Date, Double) In _
(From kvp As KeyValuePair(Of Date, Double) In _
dic Where kvp.Key >= #4/6/2011#) _
Into Sum(kvp.Value)