Clock3D is a screensaver that displays an accurate 3D clock.
If you are using a 64-bit Linux distribution, then you can simply download the latest release (currently Clock3D_v20_lin64.tar.gz), extract it, and run the executable file "Clock3D".
If you are using Windows, Mac OSX, or 32-bit Linux, you will need to compile from source. Clock3D uses CMake (Cross Platform Make). In addition to having CMake installed, make sure that you also have Irrlicht (see below) and a C++ compiler.
To compile with CMake, open up a command-line interface (cmd.exe for Windows, Terminal on Mac) and navigate to the build directory of the extracted/cloned source code (trunk/build if you cloned the SVN repository, else Clock3D_v20_src/build if you downloaded the compressed TAR file and extracted it).
Then run cmake ../src (possibly cmake ..\src on Windows). There should now be a file called "Makefile" in the build directory. If there is, run make to compile the executable.
Although Clock3D is pretty much complete, there is currently only a 64-bit Linux binary available for version 2.0, although the source code can be compiled on most operating systems and architectures. Binaries for Windows (32-bit and 64-bit) and Mac OSX are still needed.
If you feel that there are some features that Clock3D lacks, you are more than welcome to either implement them yourself or submit a feature request in the forum.
Clock3D is written in C++ and uses the Irrlicht Engine.