Sources:

• Unknown date (EDIT: according to u/tjm2000 (in reddit) it's from around 1890s): Biblioteca Virtual del Patrimonio Bibliográfico > PUERTA DEL SOL
• 2006: Puerta del Sol, Madrid, from our hotel room | Joe Calhoun | Flickr
• 2015: Plaza De La Puerta Del Sol In Madrid Spain Foto de stock - (Julian Elliott Photography) Getty Images. The 2015 date is a bit of a guess. The source does not have an explicit date, but looking at the HTML code of the page I see "dateSubmitted":"2015-11-21" so I asumed that was the year. But I'm not really sure.

Posted originally on reddit