Ryan explains how he got the date and time in the magic mirror using Geektool for Mac OS X:
This is my current desktop using Geektool with my png overlay for a dirty mirror effect above the geeklets, Dragthing with my custom theme and custom wallpaper/icons. The banners roll down and up as needed and function as docks. The mirror displays time and date and has a functioning analogue clock.
For more info, source files, and a full how-to, Ryan’s provided everything on his web site.