Keeping track of time in software sounds simle until you actually start looking into it - to take a trivial example, does a Timestamp include a TimeZone? Then there are the harder issues:

Sparking a fresh round of debate over an ongoing issue in time-keeping circles, the International Telecommunications Union is considering eliminating leap seconds from the time scale used by most computer systems, Coordinated Universal Time (UTC). Since their introduction in 1971, leap seconds have proved problematic for at least a few software programs. The leap second added on to the end of 2008, for instance, caused Oracle cluster software to reboot unexpectedly in some cases.