Funny that nobody has mentioned wanting or needing a RTC (real-time block). I guess the person with the GPS receiver doesn't need it because you get the time from that. But for a non GPS receiver, you're going to want an RTC that runs off a battery so that you don't lose the time if you lose the power to your house. I used a RTC ic that has i2c communications that I pulled off a junk circuit board I scavanged, as well as a crystal (needed for the RTC) and a CR2032 battery and holder. I have been working on building a clock, except I decided to use a Raspberry Pi instead so that I can have cheap internet access to my clock. My clock does some web requests and parses data out and scrolls them by on an OLED display and projected onto the ceiling using a jeweler's magnifying lens. I could have done this with a YUN, but in the case the Pi was at least 1/3 of the cost of a Yun.