I want a date select box with blank values selected by default

Far easier to do this:

<%= f.date_select :featured_at, {:include_blank => true, :default => nil} %>

 <%= f.label :expired %><br />
    <%= f.datetime_select :expired,:prompt => { :day => 'Select day', :month => 'Select month', :year => 'Select year' }  %>

works for me