datetime_select The helper has many different options. I never bothered to remember all of them, so I use this apidock page as a link (datetime_select and date_select have similar parameters).
In this case, you need to set the array :order :ampm to true, etc.
Edit: ok, I really bothered to write the hash option you need:
date_select("question", "answered_at", :ampm => true, :order => [:month, :day, :year], :use_short_month => true)
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