Wallpaper Clocks can be used to display various wallpapers with a real-time working clock blended into the wallpaper. Basically, the website to which I linked doesn't provide an application for it, but there are several apps which can make use of the Wallpaper Clocks provided by that website. However, the applications are not free - for Windows that is - for Linux, you can use a Screenlet which is obviously free and well, that's all you have to do. So here's how to do it.
Even though we will use Screenlets for this, the actual wallpaper won't be a screenlet, but a normal wallpaper which changes every minute or day (depending on the wallpaper you choose).