With everyone throwing down leap seconds links, here's the best one I've seen, written by the inimitable DJB: UTC, TAI, and UNIX time. Not only full of info, but also choice quotes like:
[...] POSIX is a ``standard'' designed by a vendor consortium several years ago to eliminate progress and protect the installed base. The behavior of the broken localtime() libraries was documented and turned into a POSIX requirement.
Fortunately, the POSIX rules are so outrageously dumb---for example, they require that 2100 be a leap year, contradicting the Gregorian calendar---that no self-respecting engineer would obey them.