You take it photo by photo. One photo you can be near certain it was made in a particular year, another you may be confident it's old but you're unable to determine the particular year it was made. That's why you see a cabinet card in an REA auction labeled as circa 1885 or 1860s. REA is certain they are from the advertised era, but can't give you an exact year.
News photos tend to be easier to assign to a specific year than cabinet cards, as they were for news purposes and often have the date on them. Cabinet cards are often more vague. But, yes, there will be old photos where you can't prove the "within 2 years" rule.
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