How to get tweets older than 6 months?

I am retrieving tweets using LINQ to Twitter 4.1.0 with particular hashtag. But the problem that I am facing is that i am getting only 600 tweets. I want to get tweets that are older than 6 months. Can anyone suggest me, how to do this?

[List<Search> searchResultsList = new List<Search>(); ulong lastId = 0; var context = new TwitterContext(auth); int count = 0; int maxNumberToFind = 1000;  var searchResults = (from search in context.Search where search.Type == SearchType.Search &&                                 search.Query == Query &&                                                                     search.Count == 150 &&                                 search.ResultType == ResultType.Recent &&                                 search.IncludeEntities == true select search).ToList();][1]