There are two properties for the difference in Seconds. Please see this code for an example:
private void GetTimeDiff()
{
DateTime startTime = DateTime.Parse("08:30:18");
DateTime endTime = DateTime.Parse("17:15:19");
TimeSpan t = endTime - startTime;
MessageBox.Show(t.Seconds.ToString());
MessageBox.Show(t.TotalSeconds.ToString());
}
Depending on what you want, you may use the properties "Seconds" or "TotalSeconds".