Well, everybody knows about leap years :-). What about leap seconds?
Those who are interested, can find more information on , but how it can influence your program?
First of all, you should not suppose that seconds are from 0 to 59. It could be 60 second, as well as 59 skipping and going from 58 to 00. So your system should at least be able to convert 60 second into something predictable.
Another thing that could be important for you as a developer is to know if your language support timestamps mechanisms, that ensure you uniqueness of timestamp for the particular node. For instance, in Erlang “now()” ensure you will have incrementing numbers each time you call this method.
Hopefully almost all of integrated converters has it inside the implementation. But you never know 🙂