Private Sub Workbook_Open()
If Time < TimeValue("9:30:00") Then
Application.OnTime TimeValue("9:30:00"), "Message"
ElseIf Time < TimeValue("13:30:00") Then
Application.OnTime TimeValue("13:30:00"), "Message1"
ElseIf Time < TimeValue("17:30:00") Then
Application.OnTime TimeValue("17:30:00"), "Message2"
ElseIf Time < TimeValue("21:30:00") Then
Application.OnTime TimeValue("21:30:00"), "Message3"
End If
End Sub