To answer the question, this is a CLOCK (as the heading so pointedly suggests, we'll get to BCD later). It is meant to tell the current time (from your system clock, in your timezone; if you care about that stuff). Fret not, once you understand the working of it all, this will seem little more than childs play to you!
Quite simple really, it tells time in military format (ie the 24-hour format). The first 2 columns (from your left) indicate hours, next 2 indicate minutes and final 2 show the seconds. Check it out, the last column will be updated on an every-second interval.
Using a little math, I'd say; each digit of the time HH:MM:SS is encoded into its equivalent binary format (hence the name BCD, which stands for 'Binary Coded Decimal'). For example let's say the current time is 2:30 pm which in 24-hour format is 14:30 hours, thus the binary equivalent of each digit is '0001' '0100' : '0011' '0000' (imagine the same for seconds)
This binary format is represented by the positional indicators in each column. Extending our previous example, to represent 2 pm or 14 hours or the binary '0001' '0100', our clock's hour section (first 2 columns from left, in case you forget) will glow accordingly. For '0001', first 3 indicators (from top) will be off, last one will be ON. For '0100', only 2nd indicator will be ON.
Well, since you've come so far, it'll let you in on the secret. Imagine a number in each block/indicator - First row indicators will have number 8, next row 4, next 2 and finally 1. Something like this :
| 8 | 8 | 8 | 8 | 8 | 8 |
| 4 | 4 | 4 | 4 | 4 | 4 |
| 2 | 2 | 2 | 2 | 2 | 2 |
| 1 | 1 | 1 | 1 | 1 | 1 |
Now, let us assume some of the indicators are ON, like those marked by X below :
| 8 | 8 | 8 | 8 | 8 | 8 |
| 4 | X | 4 | 4 | X | 4 |
| 2 | 2 | X | 2 | 2 | X |
| X | 1 | X | 1 | 1 | 1 |
Simply calculate the sum of ON indicators for each column, like this :
| 8 | 8 | 8 | 8 | 8 | 8 |
| 4 | X | 4 | 4 | X | 4 |
| 2 | 2 | X | 2 | 2 | X |
| X | 1 | X | 1 | 1 | 1 |
1 4 3 0 4 2
Well, there you have it, the time is 14:30:42 ie 2:30 pm (and 42 seconds!)
I have learned about and created this BCD Clock out of pure curiosity; took me a few hours to read & code it out. My point - this is not meant to be a one stop shop for all things BCDC. In fact, most representations of a BCD Clock skip the unneeded indicators (like indicators of 4 & 8 for hours). There are also different types of binary clocks; who knows, you may come up with an innovative representation all on your own!
Here are a few public domain links that might help point in proper direction -
Wanna see how this clock works behind the scene? See javascript code here
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