There are literally hundreds of applications that could use a real-time clock IC, however the variety of Arduino and compatible boards on the market can make it difficult to work with one RTC that operates over this wide spectrum of hardware. Not to be discouraged by this fact, Dave Van Wagner created his own RTC shield that works on both 3 and 5V power rails - which gives him the option of using any Arduino-compatible that accepts a standard Uno-style protoshield:
The circuitry is quite simple, and the use of a DS1307 RTC IC allows the use of a wide variety of existing Arduino libraries or simple direct I2C commands. For more information and details, visit Dave's website. And for more, we're on twitter and Google+, so follow us for news and product updates as well.