Dating a Photograph.

History The Library of Congress blog has a series of posts about the photographs in their collection and the latest entry is about dating some of the miscellaneous items by comparing them to what is or isn't there. Utilising an aerial view of their own building, they're able to show that in a city which is constantly in flux, it's entirely possible to pin it down to a decade or two:

"This photo features the Library of Congress Jefferson Building at center, and it’s always good to start with a subject you know well! The Library of Congress campus now consists of three buildings. In addition to the Jefferson Building, completed in 1897, there are the John Adams Building and the James Madison Building. In the detail photo below, the arrows point to the locations where those two buildings will be in the future. The Madison, where the Prints and Photographs Division is, will take up the entire block at lower right. The Adams will be at the right edge, where a few smaller buildings are visible. The oldest of the two is the Adams, which opened to the public January 3, 1939. So, this photo is before 1939."

Although as they say it's such a time consuming process it would be impossible to investigate every item in their collection, I wonder if there'll be a moment in the not too distant future when AI and machine learning may be able to help, following the same processes as this human.