I just finished making a clock for my work area using a 7 segment display, RTC (real time clock) and the RTC library. The nice thing about using this RTC is that it has a backup battery so if you lose power, it still maintains the time.
Fairly easy setup. Both the RTC and the 7 segment display use 12c to communicate with the Arduino. Because they have different addresses this is ok. Adafruit has this handy chart of a lot of sensors and things and what their addresses are. The 7 segment display I used has an address of 0×70 and that is not yet on the chart, but can be found in the library RTClib.
Used in this project are the Adafruit 0.56″ 4-Digit 7-Segment Display w/I2C Backpack, DS1307 Real Time Clock breakout board kit, and Adafruit Perma-Proto Half-sized Breadboard PCB.