Try like this..
string date = "18-01-2014";
string time = "10:00 am";
string timespan = "20";
double span = Convert.ToDouble(timespan);
string input = string.Format("{0} {1}",date,time);
DateTime startTime = DateTime.ParseExact(input, "dd-MM-yyyy hh:mm tt", null);
DateTime endTime = startTime.AddHours(span);
Console.WriteLine(startTime);
Console.WriteLine(endTime);