When I do a search using the top right search box, the results are always sorted by "Relevance". I have to always click on the "Sort" drop down menu and change it. In my profile settings under edit > deals > Sorting I have "Post Date/Time" selected so I'm not sure why this isn't doing what I want.
The search settings aren't the same as your deal settings. The sort "Post Date/Time" is for the deal pages only.
I don't believe you can't change the default search parameters, but you can use: "<search item> sort:created". If you feel being able to change the default search sorting is a function worth having, you can post it in the Suggestions Thread.