title: Calculation with DateTime and TimeSpan in C#
body: <p>I need to make a calculation of passed and remaining time of an operation in C#.
  I have the start of the operation saved in a string format of HH:MM:SS</p>
  
  <p>I have a default time length of the operation in a string format of HH:MM:SS</p>
  
  <p>Now I would like to calculate:</p>
  
  <ul>
  <li>The remaining time / extra time: For example if the operation is still below the default length, it should display -HH:MM:SS, and if the operation took longer than the default time, it should display +HH:MM:SS</li>
  <li>If the operation took longer, I would also like to have a double value of HH,MM in % style. For example: 3hours and 30 minutes should be displayed as 3,5</li>
  </ul>
  
  <p>Both results to be displayed next to each other.
  I know I have to translate the string values into DateTime and/or TimeSpan values to do calculations, but currently I have no idea how to calculate since the first operation for example would not give me a negative value, but just get back in time [22:30:00 of yesterday].</p>