The Hora Italica was a method of counting the hours of the day widespread in Italy, and some regions of Europe, between the fourteenth and eighteenth centuries.
The 24 hours were counted starting from half an hour after sunset: in this way, by subtracting the time read from the clock to 24, it was possible to immediately obtain how many hours of light were left.
There are still clocks on historic buildings that work on this principle.
The most famous are reproduced in the app and can also be used as an "artistic" phone wallpaper.