As we say making a clock is always fun, and size can have a lot to do with it. Many enthusiasts try to make them as small as possible - however taking to the extreme in the other direction is Arduino forum member nix_mehr_frei who designed and built a huge digital clock. They have used LED strips for each "segment" of each digit, which are easily controlled with power shift registers. The rest of the circuit is a simple Arduino and real-time clock solution which is quite simple, however the overall results are outstanding:
To learn more about making your own version, login to the Arduino forum. And we have a great tutorial on using real-time clock ICs as well. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
The most important part of any clock project is the inclusion of an accurate real-time clock IC. Here at Freetronics we have the Maxim DS3232 real-time clock IC module:
Apart from keeping accurate time for years due to the temperature-controlled oscillator and having a tiny coin-cell for backup, it is very simple to connect to your Arduino project. A driver library allows your program to easily set or read the time and date. Perfect for clock projects, dataloggers or anything that needs to know the date and time. Furthermore it contains a digital thermometer and 236 bytes of non-volatile memory to store user settings and other data. For more information, check out the module page here.