Reading a BCD clock: Add the values of each column of LEDs to get six decimal digits. There are two columns each for hours, minutes and seconds.
Here's a fun interpretation of the idea from Instructibles:
The Flock Clock uses male and female drinking birds to display time. Binary addition of the female birds (left to right) yields the hour. Binary addition of the male birds (multiplied by five) yields the minute--within five minutes. An Arduino processor provides the signals needed to drive Peltier cells which heat the fluid in the birds and causes the bobbing motion.
Here are some other Binary Clock projects by intrepid electronic experimenters, including an analog one.