string startTime = "7:00 AM";
string endTime = "2:00 PM";
TimeSpan duration = DateTime.Parse(endTime).Subtract(DateTime.Parse(startTime));
Console.WriteLine(duration);
Console.ReadKey();
Will output: 07:00:00.
It also works if the user input military time:
string startTime = "7:00";
string endTime = "14:00";
TimeSpan duration = DateTime.Parse(endTime).Subtract(DateTime.Parse(startTime));
Console.WriteLine(duration);
Console.ReadKey();
Outputs: 07:00:00.
To change the format: duration.ToString(@"hh\:mm")
More info at: http://msdn.microsoft.com/en-us/library/ee372287.aspx
Addendum:
Over the years it has somewhat bothered me that this is the most popular answer I have ever given; the original answer never actually explained why the OP's code didn't work despite the fact that it is perfectly valid. The only reason it gets so many votes is because the post comes up on Google when people search for a combination of the terms "C#", "timespan", and "between".