[...]

  Dim Conteggio As Integer
  Dim strSQL As String
  
  strSQL = "UPDATE InventarioH SET InventarioH.FlagGaranzia = 0 " & _
           "WHERE (((InventarioH.DataScadenzaGaranzia)<=Fix(Now())));"

  DoCmd.RunSQL strSQL

  Conteggio = DCount("IDMatricola", "InventarioH", "[DataScadenzaGaranzia]-30 <= Fix(Now()) AND [FlagGaranzia]=True")

  If Conteggio = 0 Then
    DoCmd.Close
    DoCmd.OpenForm "login"
  End If

[...]