Answer to http://stackoverflow.com/questions/43527509/c-sharp-linq-to-xml-element

Foo.cs

using System;
using System.IO;
using System.Linq;
using System.Xml.Linq;

namespace ConsoleApplication1
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var doc = XDocument.Load(new StringReader(RawXml));
            DateTime testDate = new DateTime(2017, 03, 01);

            var dateElements = doc.Descendants("NewDataSet")

                // gather all the child-elements of all NewDataSet elements
                .SelectMany(dataSetEle => dataSetEle.Elements())

                // filter out child-elements that themselves have no applicable DATE child-elements
                .Where(childEle => childEle.Elements()
                    .Any(grandChildEle =>
                        grandChildEle.Name == "DATE" && (DateTime) grandChildEle > testDate)
                );

            foreach (var dateElement in dateElements)
            {
                Console.WriteLine(dateElement);
            }

            Console.WriteLine("PRESS KEY TO END");
            Console.ReadKey();
        }

        private static readonly string RawXml = @"<DataSet>
   <diffgram xmls="""">
       <NewDataSet>
          <USER_LOGIN>
              <USER_LOGIN_ID>123</USER_LOGIN_ID>
              <DATE>2017-03-01T16:56:16.59-06:00</DATE>
          </USER_LOGIN>
          <CONTENT>
              <CONTENT_ID>123</CONTENT_ID>
              <DATE>2017-03-01T16:56:16.59-06:00</DATE>
          </CONTENT>
        <SHOULDBEIGNORED>
              <CONTENT_ID>123</CONTENT_ID>
          </SHOULDBEIGNORED>
        <SHOULDBEIGNORED>
              <CONTENT_ID>123</CONTENT_ID>
              <DATE>2000-03-01T16:56:16.59-06:00</DATE>
          </SHOULDBEIGNORED>
       </NewDataSet>
   </diffgram> 
</DataSet>";
    }
}