A named day, by which an event can be placed in time. A date is what makes a claim checkable against a record.
Latin data (given), from dare (to give), taken from the Roman letter-closing convention data Romae (given at Rome, on such a day); a named day by which an event can be placed, and later checked, in time.
The Roman letter-writer's own habit is preserved whole: a date was the day a document said it was given, so a later reader could check the claim against the calendar. It is a checkpoint borrowed from correspondence, not a natural unit of time.