I had this clock for few years
and it always drove me crazy how individual LEDs had different brightness as you can tell from the photo above. Not to mention that they were not exactly precise in time keeping.
Every digit was made from 13 LEDs, I added 2 more to fill "holes" in 8 to form small 3x5 LED matrix for every digit.
To get idea how finished clock works please take a look at video bellow