Cheers, would i need to add a namespace at the top?

I am getting the following errors:-

Error 2 The type or namespace name 'CultureInfo' could not be found (are you missing a using directive or an assembly reference?) 

Error 5 Argument '2': cannot convert from 'lambda expression' to 'System.Func<System.Xml.Linq.XElement,System.DateTime>

Error 4 'System.Collections.Generic.IEnumerable<System.Xml.Linq.XElement>' does not contain a definition for 'OrderByDescending' and the best extension method overload 'System.Linq.Enumerable.OrderByDescending<TSource,TKey>(System.Collections.Generic.IEnumerable<TSource>, System.Func<TSource,TKey>)' has some invalid arguments