You do not need to convert this to a string.
DateTime start = DateTime.Now;
DateTime end = DateTime.Now;
(Note: Those two times above will be identical)
Once you have done that, you can use one of the other techniques shown above to get the timespan:
var timeSpent = (end - start);
or
TimeSpan timeSpent = end.Subtract(start);
To display it:
Console.WriteLine(timeSpent.TotalMilliseconds);
Now, go code! :)