I presume the data is 12th march. in that case in my version and regional configuration it should be entered as 3/12/09.
it is entered in A!
right click the sheet tab and click view code and in the resulting window copy paste this EVENT CODE. whenever you activate this sheet this event code is invoked. If 6 months(180 days) are over then B11 will have entry "expired" and A1 will be in red.
Private Sub Worksheet_Activate()
Dim x As Integer
Range("A1").Interior.ColorIndex = xlNone
x = Date - Range("A1")
If x > 180 Then
Range("B1") = "expired"
Range("A1").Interior.ColorIndex = 3