Hi Cindy, 'Dated' meaning 'outmoded' is certainly not incorrect, but probably much the more common usage, outside of the set phrase 'this letter, dated 2 January'. While Oxford calls it colloquial, Webster's makes no such distinction. As to its history, I can only say that use is not mis-use if followed by a considerable number of native speakers: if that is what it has come to mean, then that is the normal evolution of the idiom.
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