SELECT SUM(TaskLength) as TaskLength FROM myTable
                  WHERE EventStartTime
                  BETWEEN '2019/8/17' AND '2019/8/19'
                  GROUP BY TaskName ORDER BY TaskLength

TaskName        TaskLength
Idle Time       20
Start Shift     31
Downtime        85
Engineering     120
Part Prep       141
Maintenance     172
Production      417

using (SqlConnection con = new SqlConnection(_connectionString))
  {
   string query = @"SELECT SUM(TaskLength) as TaskLength FROM myTable
                  WHERE EventStartTime
                  BETWEEN '2019/8/17' AND '2019/8/19'
                  GROUP BY TaskName ORDER BY TaskLength";

   using (SqlCommand cmd = new SqlCommand(query, con))
        {
          con.Open();
          object result = cmd.ExecuteScalar();
          string totalMinutes = Convert.ToString(result);
        }
 }