This ,circuit as such, is arguably the most difficult and extensive project I have ever done till date. In fact, it actually took me 2 meters of wires for the connections, almost 17 IC’s, 3 days of continuous working and not to mention a month of Boolean calculations. But then and again, I enjoyed every second of it!
I posted the details of the construction along with the circuit itself at my Instructables page (another blog like page of mine where step by step instructions of projects will be there): http://www.instructables.com/id/Digital-Clock-Sequential-Logic-Design/
This blog, however, is aimed only to tell what one can find in that link.
While most other hobbyists use micro-controllers to build digital clocks, I went a step backward (more basic) and did the same using flip flops. Micro-controllers themselves contain a lot of flip-flops. But what is a flip-flop and what are the different types? How do you design a circuit with them? I have answered all these questions in that link.
I have literally covered sequential logic design by using the digital clock and its various sub-circuits as examples. It even has an alarm function. It is difficult compared to the alternate ways like Arduino etc., but it’s fun if you love electronics!