The leap second bug

The thing is that you can only get UTC h:mm:ss and local timezone seconds using modulus on time_t, due to half-hour-offset timezones, DST and irregular month lengths.

Now, you generally either want to display UTC date+time or local h:mm:ss at least, neither of which you can do without calling library functions, so maybe not much or anything will break?

I'm not sure I'd risk it though, and not adding leap seconds ever again to UTC seems a much better solution.