For the LCC project I had to add a real time clock (RTC) so that it could manage the light schedule independently of a PC. I added a fairly generic RTC shield. I could have used this one or this one. They both use the I2C bus and the DS1307 RTC chip.
The example from LadyAda didn’t allow me to manipulate the date/time components well enough so I went with a library that pulls the date/time in BCD format. The issue was that I needed those preceding zeros on numbers less than 10. 8:8:8 is not as easy to read as 08:08:08 to me. The library I used allowed me to prepend the zero when necessary.
The library is from the Arduino Playground and also has lots of time related functions that work with any DS1307 chip.