Just like a binary clock. NICE!
The problem is, a lot of people wimp-out, and implement a binary clock that is really a BCD (binary-coded decimal) clock.
Binary-coded decimal is very wasteful (of bits).
BCD reserves four bits for the numbers 0..9.
So 19 decimal is represented as 00011001
(to give you an idea of how wasteful BCD is, it takes eight bits to represent the numbers 0..99, whereas in binary, those eight bits could represent numbers 0..255. The problem gets worse the more decimal digits you add)
I'm up to number 132 so far.
132 is 128 + 4, so 10000000 + 00000100 = 10000100
I'm afraid you're going to come across two's complement pretty soon...stay clam.