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If you have used Twitter search before, you may notice that you can only go back a certain amount of time and/or number of tweets for a given search. In fact, if you read the Twitter search documentation, you'll note that the folks from Twitter say, "We also restrict the size of the search index by placing a date limit on the updates we allow you to search. This limit is currently around a month but is dynamic and subject to shrink as the number of tweets per day continues to grow."