UTC is different from epoch time. Epoch time by definition does not count leap seconds. https://en.m.wikipedia.org/wiki/Unix_time
It sounds like what it means is that Unix time counts the number of real, actual, by-the-clock seconds that have passed since the epoch. That would be logical. But what it actually means is that it counts the number of real, actual, by-the-clock seconds, minus the number of those those that have been designated "leap seconds".
That is to say, whenever a "leap second" occurs, the nice monotonic isotonic progress of unix time is mutilated by suddenly adding or removing 1 to the total count so far. That's what "does not count leap seconds" means, and sometimes even what "ignores leap seconds" means (which is of course even worse terminology).
> The time() function will resolve to the system time of that server. If the server is running an NTP daemon then it will be leap second aware and adjust accordingly. PHP has no knowledge of this, but the system does.
> It took me a long time to understand this, but what happens is that the timestamp increases during the leap second, and when the leap second is over, the timestamp (but not the UTC!) jumps back one second.
[1] http://stackoverflow.com/questions/7780464/is-time-guarantee...
