Try it like this
Private Sub Btn_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Btn_Add.Click, Btn_Sub.Click
Dim btn_clicked As Button = CType(sender, Button)
Static startTime As DateTime = #12/30/5000 4:30:00 AM#
If btn_clicked.Name = "Btn_Add" Then
startTime = startTime.AddMinutes(15)
Label1.text = startTime.ToString("hh:mm tt")
ElseIf btn_clicked.Name = "Btn_Sub" Then 'in the OP this said Btn_Add also
startTime = startTime.AddMinutes(-15)
Label1.text = startTime.ToString("hh:mm tt")
End If
End Sub