Customizable layout of a whole day in your current timezone and as many other timezones as you like. Also shows the current hour. This is really good when an online friend says something will be at 10am her time and you need to know when to be awake in your timezone!
Nano Apps are very very small browser apps - so small, in fact, that they have absolutely no server-side code at all. Hosting is 100% static files. You could run them in your browser from your local file system.
But what makes them apps, then?
Client-side JavaScript. Hand written, not generated. No frameworks. Even the HTML markup and styles are hand-written. This is all what makes then Nano Apps. I mean, "nano" is smaller than "micro"... I will admit more than about 50 lines of Javascript starts to need some serious discipline and structure, but it's very possible.
Okay, there might be some third-party libraries, mostly to do things that JavaScript is not good at. Like date handling. Currently, the preference for date handling is Luxon which is small and non-intrusive.