If you are interested in building your very own clock, you should also take a look at the
"Doomsday Clock Shield Kit"
http://samuraicircuits.com/merch/index.php?id_category=14&controller=category
It is designed to work with the "Extremely Accurate Real Time Clock". This way you can build a clock that can keep very accurate time. According to the datasheet it will lose less than one minute per year.