日期时间减法

我的注册程序在人们扫描自己时会使用他们的姓名、ID 和"Time In："登录并隐藏他们，但在他们的名字中添加"Time Out："，反之亦然。我想做的是每次这个人扫描自己"进入"时，我希望它查看时间"进入"和"输出"并计算在办公室的总时间。

附上代码：

    private string CreateTimeEntry(string current)
    {
        var indexIn = current.LastIndexOf("Time In : "); // Get the last index of the word "in"
        var indexOut = current.LastIndexOf("Time Out : "); // Get the last index of the word out
        string timeIn = current + "      " + "Time In : ";
        string timeOut = current + "      " + "Time Out : ";
        if (indexOut > indexIn)
        {
            // if the last "out" comes after the last "in"
            return timeIn;
        }
        else
        {
           // If the last "in" comes after the last "out"
            return timeOut;
        }
    }
    private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
    {
        SqlConnection DBConnection = new SqlConnection("Data Source=DATABASE;Initial Catalog=imis;Integrated Security=True");
        SqlCommand cmd = new SqlCommand();
        Object returnValue;
        string txtend = textBox1.Text;
        if (e.KeyChar == 'L')
        {   
            DBConnection.Open();
        }
        if (DBConnection.State == ConnectionState.Open)
        {
            if (textBox1.Text.Length != 6) return;
            {
                cmd.CommandText = ("SELECT last_name +', '+ first_name from name where id =@Name");
                cmd.Parameters.Add(new SqlParameter("Name", textBox1.Text.Replace(@"L", "")));
                cmd.CommandType = CommandType.Text;
                cmd.Connection = DBConnection;
                returnValue = cmd.ExecuteScalar() + "'t (" + textBox1.Text.Replace(@"L", "") + ")";
                DBConnection.Close();
                bool found = false;
                System.DateTime resultTime1 = new System.DateTime(;
                foreach (var item in listBox1.Items)
                {
                    var itemEntry = item.ToString();
                    string newEntry = CreateTimeEntry(itemEntry) + DateTime.Now.ToString("HH:mm") + "   " + "Total Time: " + resultTime1 ;
                    if (itemEntry.Contains(returnValue.ToString()))
                    {
                        var indexIn = itemEntry.LastIndexOf("Time In : ");
                        var indexOut = itemEntry.LastIndexOf("Time Out : ");
                        if (indexOut > indexIn)
                        {
                            listBox2.Items.Remove(item);
                            listBox1.Items.Add(newEntry);
                            found = true;
                            break;
                        }
                        else
                        {
                            listBox1.Items.Remove(item);
                            listBox2.Items.Add(newEntry);
                            found = true;
                            break;
                        }
                    }
                }
                if (!found)
                {
                    string newEntry2 = "";
                    foreach (string str in listBox2.Items)
                    {
                        var itemEntry2 = str;
                        newEntry2 = CreateTimeEntry(itemEntry2) + DateTime.Now.ToString("HH:mm");
                        //if (listBox2.Items.Contains(returnValue.ToString()))
                        if (listBox2.Items.Contains(str) && str.Contains(textBox1.Text))
                        {
                            var indexIn = itemEntry2.LastIndexOf("Time In : ");
                            var indexOut = itemEntry2.LastIndexOf("Time Out : ");
                            if (indexOut > indexIn)
                            {
                                listBox2.Items.Remove(str);
                                listBox1.Items.Add(newEntry2);
                                found = true;
                                break;
                            }
                        }
                    }
                    var itemEntry = listBox1.Items.ToString();
                    var itemEntry1 = listBox2.Items.ToString();
                    if (!listBox1.Items.Contains(newEntry2))
                    {
                        listBox1.Items.Add(returnValue + "      " + "Time In : " + DateTime.Now.ToString("HH:mm"));
                    }
                }
            }
            textBox1.Clear();
            System.IO.StreamWriter SaveFile = new System.IO.StreamWriter(fullFileName);
            foreach (object item1 in listBox1.Items)
                SaveFile.WriteLine(item1.ToString());
            SaveFile.Flush();
            SaveFile.Close();
            if (listBox1.Items.Count != 0) { DisableCloseButton(); }
            else
            {
                EnableCloseButton();
            }
            Current_Attendance_Label.Text = "Currently " + listBox1.Items.Count.ToString() + " in attendance.";
            e.Handled = true;
        }