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When a project is set to sort by date, Squirro will sort the result list by using the created_at field of each item i.e. the most recent documents are put first in the result list. The limitation of of sorting this way is that the most recent documents are often poor matches for the original query. See Combined Time and Relevance Sorting - Smart Rank for details on trade-offs.
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Squirro search results can be sorted in by Date as default on a per-project basis by using setting in the Advanced Options.