Sub test()
    Dim i As Integer, Lastrow As Integer
    Dim MyDate As Variant
    Dim myDay As Integer
    
    'get the lastrow with data in it
    Lastrow = Range("D" & Rows.Count).End(xlUp).Row
    
    For i = 1 To Lastrow
        MyDate = Range("D" & i).Formula
        'try to convert to date to see if this works
        On Error Resume Next
        MyDate = CDate(MyDate)
        'if error occurs display message and write error on the worksheet
        If Err.Number <> 0 Then
            MsgBox "cell D" & i & " is not a date"
            Range("E" & i).Formula = "Error" & Error(Err.Number)
            On Error GoTo 0
        Else
            'otherwise put formula's on worksheet dependent on the day
            On Error GoTo 0
            'get day from the date
            myDay = Day(MyDate)
            Select Case myDay
                Case 1 To 10
                    'do something here i will put in a
                    Range("E" & i).Formula = "a"
                Case 11 To 20
                    Range("E" & i).Formula = "b"
                Case 21 To 31
                    Range("E" & i).Formula = "c"
            End Select
        End If
    Next i
End Sub