Как я считаю правильное время, когда дата меняется VB

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я делаю программу для своей работы, чтобы рассчитать часы, отработанные материалом, у меня есть 2 текстовых поля для каждого дня недели, одно для того, чтобы поставить время начала и следующее Время окончания, я заставляю его работать без проблем, пока не поставлю время окончания до 24: 00 или 24: 30
или 00: 00 или 00: 30

Что я уже пробовал:

Dim a As String
        Dim t1 As DateTime '= DateTime.Parse(stathisa.Text)
        Dim t2 As DateTime '= DateTime.Parse(stathis1.Text)
        Dim t3 As DateTime '= DateTime.Parse(stathisb.Text)
        Dim t4 As DateTime '= DateTime.Parse(stathis2.Text)
        Dim t5 As DateTime '= DateTime.Parse(stathisc.Text)
        Dim t6 As DateTime '= DateTime.Parse(stathis3.Text)
        Dim t7 As DateTime '= DateTime.Parse(stathisd.Text)
        Dim t8 As DateTime '= DateTime.Parse(stathis4.Text)
        Dim t9 As DateTime '= DateTime.Parse(stathise.Text)
        Dim t10 As DateTime '= DateTime.Parse(stathis5.Text)
        Dim t11 As DateTime '= DateTime.Parse(stathisf.Text)
        Dim t12 As DateTime '= DateTime.Parse(stathis6.Text)
        Dim t13 As DateTime '= DateTime.Parse(stathisg.Text)
        Dim t14 As DateTime '= DateTime.Parse(stathis7.Text)




        
       
        If stathisa.Text = "OFF" Or stathis1.Text = "OFF" Then
            stathis1.Text = "OFF"
            stathisa.Text = "OFF"
           
        End If

        If stathisb.Text = "OFF" Or stathis2.Text = "OFF" Then
            stathis2.Text = "OFF"
            stathisb.Text = "OFF"
           
        End If

        If stathisc.Text = "OFF" Or stathis3.Text = "OFF" Then
            stathis3.Text = "OFF"
            stathisc.Text = "OFF"

        End If
        If stathisd.Text = "OFF" Or stathis4.Text = "OFF" Then
            stathis4.Text = "OFF"
            stathisd.Text = "OFF"

        End If
        If stathise.Text = "OFF" Or stathis5.Text = "OFF" Then
            stathis5.Text = "OFF"
            stathise.Text = "OFF"

        End If
        If stathisf.Text = "OFF" Or stathis6.Text = "OFF" Then
            stathis6.Text = "OFF"
            stathisf.Text = "OFF"
            ''
        End If
        If stathisg.Text = "OFF" Or stathis7.Text = "OFF" Then
            stathis7.Text = "OFF"
            stathisg.Text = "OFF"

        End If
       
        If stathisa.Text = "HOLIDAYS" Or stathis1.Text = "HOLIDAYS" Then
            stathis1.Text = "HOLIDAYS"
            stathisa.Text = "HOLIDAYS"

        End If

        If stathisb.Text = "HOLIDAYS" Or stathis2.Text = "HOLIDAYS" Then
            stathis2.Text = "HOLIDAYS"
            stathisb.Text = "HOLIDAYS"

        End If

        If stathisc.Text = "HOLIDAYS" Or stathis3.Text = "HOLIDAYS" Then
            stathis3.Text = "HOLIDAYS"
            stathisc.Text = "HOLIDAYS"

        End If
        If stathisd.Text = "HOLIDAYS" Or stathis4.Text = "HOLIDAYS" Then
            stathis4.Text = "HOLIDAYS"
            stathisd.Text = "HOLIDAYS"

        End If
        If stathise.Text = "HOLIDAYS" Or stathis5.Text = "HOLIDAYS" Then
            stathis5.Text = "HOLIDAYS"
            stathise.Text = "HOLIDAYS"

        End If
        If stathisf.Text = "HOLIDAYS" Or stathis6.Text = "HOLIDAYS" Then
            stathis6.Text = "HOLIDAYS"
            stathisf.Text = "HOLIDAYS"
            ''
        End If
        If stathisg.Text = "HOLIDAYS" Or stathis7.Text = "HOLIDAYS" Then
            stathis7.Text = "HOLIDAYS"
            stathisg.Text = "HOLIDAYS"

        End If





       
        If stathisa.SelectedIndex = -1 Or stathis1.SelectedIndex = -1 Then
            stathis1.SelectedIndex = -1
            stathisa.SelectedIndex = -1

        End If

        If stathisb.SelectedIndex = -1 Or stathis2.SelectedIndex = -1 Then
            stathis2.SelectedIndex = -1
            stathisb.SelectedIndex = -1

        End If

        If stathisc.SelectedIndex = -1 Or stathis3.SelectedIndex = -1 Then
            stathis3.SelectedIndex = -1
            stathisc.SelectedIndex = -1

        End If
        If stathisd.SelectedIndex = -1 Or stathis4.SelectedIndex = -1 Then
            stathis4.SelectedIndex = -1
            stathisd.SelectedIndex = -1

        End If
        If stathise.SelectedIndex = -1 Or stathis5.SelectedIndex = -1 Then
            stathis5.SelectedIndex = -1
            stathise.SelectedIndex = -1

        End If
        If stathisf.SelectedIndex = -1 Or stathis6.SelectedIndex = -1 Then
            stathis6.SelectedIndex = -1
            stathisf.SelectedIndex = -1
            ''
        End If
        If stathisg.SelectedIndex = -1 Or stathis7.SelectedIndex = -1 Then
            stathis7.SelectedIndex = -1
            stathisg.SelectedIndex = -1

        End If

       


       
        If stathisa.Text.Contains("OFF") = False Or
           stathisa.Text.Contains("HOLIDAYS") = False Or
           stathisa.Text = vbNullString = False Or
           stathis1.Text.Contains("OFF") = False Or
           stathis1.Text.Contains("HOLIDAYS") = False Or
           stathis1.Text = vbNullString Then

            Try




                t1 = DateTime.Parse(stathisa.Text)
                t2 = DateTime.Parse(stathis1.Text)

            Catch ex As Exception

            End Try



        Else

            t1 = vbNullString

            t2 = vbNullString
        End If







        If stathisb.Text.Contains("OFF") = False Or
       stathisb.Text.Contains("HOLIDAYS") = False Or
       stathisb.Text = vbNullString = False Or
       stathis2.Text.Contains("OFF") = False Or
       stathis2.Text.Contains("HOLIDAYS") = False Or
       stathis2.Text = vbNullString Then

            Try




                t3 = DateTime.Parse(stathisb.Text)
                t4 = DateTime.Parse(stathis2.Text)

            Catch ex As Exception

            End Try

        Else
            t3 = vbNullString

            t4 = vbNullString
        End If




        If stathisc.Text.Contains("OFF") = False Or
   stathisc.Text.Contains("HOLIDAYS") = False Or
   stathisc.Text = vbNullString = False Or
   stathis3.Text.Contains("OFF") = False Or
   stathis3.Text.Contains("HOLIDAYS") = False Or
   stathis3.Text = vbNullString Then

            Try

           
            t5 = DateTime.Parse(stathisc.Text)
                t6 = DateTime.Parse(stathis3.Text)


            Catch ex As Exception

            End Try

        Else
            t5 = vbNullString
            t6 = vbNullString

        End If







        If stathisd.Text.Contains("OFF") = False Or
stathisd.Text.Contains("HOLIDAYS") = False Or
stathisd.Text = vbNullString = False Or
stathis4.Text.Contains("OFF") = False Or
stathis4.Text.Contains("HOLIDAYS") = False Or
stathis4.Text = vbNullString Then


            Try

           
            t7 = DateTime.Parse(stathisd.Text)
                t8 = DateTime.Parse(stathis4.Text)

            Catch ex As Exception

            End Try

        Else
            t7 = vbNullString
            t8 = vbNullString

        End If





        If stathise.Text.Contains("OFF") = False Or
stathise.Text.Contains("HOLIDAYS") = False Or
stathise.Text = vbNullString = False Or
stathis5.Text.Contains("OFF") = False Or
stathis5.Text.Contains("HOLIDAYS") = False Or
stathis5.Text = vbNullString Then

            Try

            
            t9 = DateTime.Parse(stathise.Text)
                t10 = DateTime.Parse(stathis5.Text)

            Catch ex As Exception

            End Try


        Else
            t9 = vbNullString
            t10 = vbNullString

        End If





        If stathisf.Text.Contains("OFF") = False Or
stathisf.Text.Contains("HOLIDAYS") = False Or
stathisf.Text = vbNullString = False Or
stathisf.Text.Contains("OFF") = False Or
stathisf.Text.Contains("HOLIDAYS") = False Or
stathisf.Text = vbNullString Then

            Try

            
            t11 = DateTime.Parse(stathisf.Text)
                t12 = DateTime.Parse(stathis6.Text)


            Catch ex As Exception

            End Try




        Else
            t11 = vbNullString
            t12 = vbNullString

        End If







        If stathisg.Text.Contains("OFF") = False Or
stathisg.Text.Contains("HOLIDAYS") = False Or
stathisg.Text = vbNullString = False Or
stathis7.Text.Contains("OFF") = False Or
stathis7.Text.Contains("HOLIDAYS") = False Or
stathis7.Text = vbNullString Then



            Try

            
            t13 = DateTime.Parse(stathisg.Text)
                t14 = DateTime.Parse(stathis7.Text)

            Catch ex As Exception

            End Try


        Else
            t13 = vbNullString
            t14 = vbNullString

        End If
        


        


        Dim ts As TimeSpan = ((t1.Subtract(t2)) + (t3.Subtract(t4) + (t5.Subtract(t6) + (t7.Subtract(t8) + (t9.Subtract(t10) + (t11.Subtract(t12) + (t13.Subtract(t14))))))))
        a = (ts).Hours.ToString("00") & ":" & ts.Minutes.ToString("00")
        a = Abs(ts.TotalHours)
        TextBox1.Text = a

        If a > 42 Then
            TextBox1.ReadOnly = False
            TextBox1.ForeColor = Color.Red
            MessageBox.Show("Maximum Hours Exceeded")
        Else
            TextBox1.ForeColor = Color.Black
            TextBox1.ReadOnly = True

        End If

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