I went looking for a good clock screen saver for Mac OS X today, found a few but figured I could probably create my own. After considering what tool to use I decided I'd give Quartz Composer a go. Turns out it was both easy and hard.
Creating the basic clock was not very hard, getting it to not flicker was a bit harder (it appears that this is a bug with how Quartz Composer files are rendered as screensavers).
What I ended up doing was creating an Interpolation patch that feeds into the Hue input of an HSL Color patch with its Luminosity value set to 0.001 which in turn acts as the background for the clock text, this way it appears like the screen is in constant need of being redrawn even when it is not, thus avoiding the flicker issue I had.
I went for a very large font (Helvetica Neue Bold covering almost the entire screen) but this can be easily changed by opening the .qtz file with Quartz Composer and editing the Image With String patch.
To install it all you have to do is copy/move the .qtz file to ~/Library/Screen Savers/. It should now show up in your list of installed screen savers.