While reading a history of timepieces, I learned that features a watch performs – above and beyond telling the time – are known as complications. A complication can be a calendar date, alarms, lunar phase etc. Apple too, refers to apps for the Apple Watch as complications.
I suppose it comes from a time where watch makers were competing on features and wanted to draw attention to the mechanisms and engineering that powered these complications. Perhaps an inside joke into the extra effort they caused.
Response to Daily Prompt: Complication.