The answer is simple, and very clever indeed: by not concentrating on the product - something cannot be both old and modern at the same time, but by focusing on the packaging instead:
|Old Dutch Cleanser, Saturday Evening Post, 1939|
Or, like they did the year before, by focusing on the buyer:
|Old Dutch Cleanser,1938|
A very clever bit of advertising ...
 Modern Living series, for more info follow this link