string dateString = "November 16 2015 20:15:00";
string format = "MMMM dd yyyy HH:mm:ss";
DateTime itemdate = DateTime.ParseExact(dateString, format, CultureInfo.InvariantCulture);
TimeSpan span = itemdate.Subtract(new DateTime(1970,1,1,0,0,0));
return span.TotalSeconds;
и директивы надо...