Loop through data set and use dateDiff in VB.net

I am trying to loop through a DataSet table and find the difference for some dates.  I keep getting  invalid DBnull errors.  When I try to set my dims to DBnull the code still won't work.  Here is my code:

Private Sub censusFinalTable()
        Dim Conn_PatientDaysDlyCY As SqlConnection
        Dim SqlSelect_PatientDaysDlyCY As SqlCommand
        Dim DSfinal As New DataSet()
        Dim myreaderCY As SqlDataReader

        Dim ConnString As String = "*********************************************************************"
        Dim SelectStatementCY As String = "SELECT CensusRdPhyBcd.Account AS Expr1, vw_ThomasData.fc, vw_ThomasData.admdate, vw_ThomasData.disdate" + _
                                          " FROM CensusRdPhyBcd LEFT OUTER JOIN " + _
                                          "vw_ThomasData ON CensusRdPhyBcd.Account = vw_ThomasData.acctno" + _
                                          " ORDER BY vw_ThomasData.admdate"

        Dim adaptor As SqlDataAdapter = New SqlDataAdapter(SelectStatementCY, ConnString)
        Dim cmdBuilder As SqlCommandBuilder = New SqlCommandBuilder(adaptor)

        Conn_PatientDaysDlyCY = New SqlConnection(ConnString)
        Conn_PatientDaysDlyCY.Open()
        SqlSelect_PatientDaysDlyCY = New SqlCommand(SelectStatementCY, Conn_PatientDaysDlyCY)


        myreaderCY = SqlSelect_PatientDaysDlyCY.ExecuteReader
        adaptor.Fill(DSfinal)

        '******************************************************************************************
     
        Dim firstDay As Date
        Dim lastDay As Date
        Dim days As String
        Dim admdate As String




        Dim disDate As String



        Dim aditdate As DBNull
        Dim DischargeDate As DBNull






        'loop through dataset
        Dim J As Integer
        aditdate = DSfinal.Tables(0).Rows(J).Item(2)
        DischargeDate = DSfinal.Tables(0).Rows(J).Item(3)

        admdate = aditdate

        For J = 0 To DSfinal.Tables(0).Rows.Count - 1
            If admdate <= "04/01/2007" Then
                firstDay = "04/01/2007"

            Else
                firstDay = admdate

                If DisDate >= "04/30/2007" Then
                    lastDay = "04/30/2007"
                Else
                    lastDay = DisDate

                End If

                days = DateDiff("d", firstDay, lastDay)
                MsgBox(lastDay)


            End If
        Next

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