This is currently built for OSX, but with minimal modification could be used on any *NIX system.

The tod.sh script is what changes your desktop background based on the time of day. However in order for this to be automatic, you must set it up to run via cron. The following crontab entry works on OSX 10.8.5, and should work with no modification for any system.

First, execute EDITOR=nano crontab -e from the command line. Add a line to the bottom that matches the following, but replace the path with the path to the tod.sh script on your own machine. Vim does not work to edit crontab on OSX for some reason, hence the EDITOR=nano.

0 * * * * /Users/Paul/Repositories/8bit-background/tod.sh

This will schedule the script to run once an hour.

By default, tod.sh sets the background for your primary monitor. If you want it to set the background of a particular monitor, pass the -m flag with the integer value representing the monitor you want to set. This is based on the order in which your desktops are configured in OSX. Passing 1 will set your primary monitor, 2 your secondary monitor, and so forth. You can set the desktop on all available monitors with the -a flag.

These images come from Pokemon Sapphire (or that's what I've been led to believe by Reddit). If you are the original author and would like credit, please send me a message so I can properly credit you for your work.