This clock has been mentioned in the news recently. It’s the Set Theory Clock, also known as the Berlin Clock. There’s an excellent description — and a working version — at 3Quarks:Berlin Clock. It consists of 24 lights (and a 25th one on the top which flashes every second).
From the description at 3Quarks:
It makes use of the principle of set theory to depict the time. The time of day is displayed in a 24-hour format and can be determined by simply adding and multiplying the glowing lights.
The first, uppermost row consists of 4 red lights, whereby each of these lights stands for 5 full hours. The 4 red lights in the second row display one full hour apiece. For example, if the first 2 lights in the uppermost row and all 4 lights in the second row are lit up, that represents 1400 hours, or 2 p.m. (2 × 5 + 4 hours).
The third row is composed of 11 lights: 3 red and 8 yellow. Each light in this row stands for 5 elapsed minutes. The 3 red lights have been assigned to mark the quarters of an hour and are intended to make reading the clock easier. Last of all, the yellow row at the very bottom displays units of single minutes.
The working version can be found at 3Quarks – Set Theory Clock.
The original version was installed in 1975:
Dieter Binninger, an inventor and tinkerer from Berlin who is also a trained clockmaker, designed the Set Theory Clock on behalf of the Berlin Senate in 1975. The clock was installed on Kurfürstendamm in the Berlin-Charlottenburg district and rapidly evolved into a tourist attraction, though it turned out to have one serious disadvantage: its inner workings consisted of hundreds of light bulbs, some of which constantly burned out.
For clock watchers, the clock shows different time scales. You can observe the slow march of the 5-hour periods on the top bar, or watch each minute pass by on the bottom bar. Unfortunately, reading the precise time requires some mental arithmetic, which makes the clock hard to use.
As for why it’s in the news, some references to a “Berlin Clock” have been made in reference to the famous Kryptos sculpture: Sculptor Offers Another Clue in 24-Year-Old Mystery at C.I.A..
If it’s not a reference to this clock, perhaps it’s a reference to the Alexanderplatz World Clock: