Having no date marked on it, or not assigned to a specific time period.
From un- (not) plus dated, from Latin data meaning 'given' or 'assigned.' The concept of dating documents became crucial with the development of record-keeping and legal systems.
Undated documents are like temporal orphans - they float free from the anchoring of time, which makes them either mysteriously timeless or frustratingly unreliable, depending on your perspective as historian or romantic.
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