The solution here is that any software that relies on accurate timing and/or breaks when you change the time should be using epoch seconds, not any sort of human-oriented time format.
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...
- UTC timestamps can unambiguously refer to times years in the future; TAI timestamps cannot, because it is unknown how many seconds will be in each year.
- Converting UTC timestamps to human-readable UTC times is simple modular arithmetic. A beginner in any programming language can do it. Converting TAI to UTC requires a lookup table, and it must be updated after the software is released.
What would be simpler would be ending the use of leap seconds for a millennium or so.
TAI timestamps don't refer unambiguously to UTC timestamps or calendar dates in the future, because the latter two depend on the variable rotation of the earth and (for zoned times) geopolitical whimsy.
I don't see why this matters though - most "timestamps" are for events in the past. The proper representation for events in the future will depend on your application (eg. are you writing a calendar for humans, or a spacecraft guidance system - does the event happen at a fixed point in time or a fixed point in the human work day?).
UTC doesn't know how long a second is, and TAI doesn't know how long a day or a year is. But most people need to specify "10 years from now" more often than they need to specify "300 megaseconds from now".
Banning leap seconds would be fine. UTC leap seconds are messy but at least we get by. But updating all time conversion software (instead of just updating authoritative clocks) every time there's a leap second is ludicrous.
It's true that applications should use the proper representation for what they're intended to do. And they do. Most applications use UTC because they describe human-centric events. Astronomers use TAI.
In my opinion, the real problem is that TAI is not an option in most current systems. There is no way to get the time in TAI, no way to convert between TAI and UTC etc.
So even in applications where it makes sense to use TAI (think logging and billing) we don't do that because the necessary infrastructure is not available.
I think it is time that the technical community gets together can make TAI a first class citizen.
TAI doesn't help with timestamps in the future, but usually those applications don't need second level granularity anyhow. The applications that break during a leap second are the ones that need to track the current time or passage of time with sub-second accuracy. And those can be served perfectly with TAI.
> But updating all time conversion software (instead of just updating authoritative clocks) every time there's a leap second is ludicrous.
All time conversion software is already updated every time a government changes a time zone - by downloading the most recent tzdata. All software that needs second-level granularity is constantly updated, by synchronizing with NTP. There's nothing at all ludicrous about distributing leap second tables instead of mutilating the NTP time signal.
Isn't it the other way around, since TAI has no leap seconds?
"What would be simpler would be ending the use of leap seconds for a millennium or so."
That effectively means using TAI consistently, which is what software not aware of leap seconds would be doing anyway (despite the fact that it's actually working with UTC.)
And yes, I'm advocating for changing that definition, and making UTC a constant offset from TAI that works the same as TAI for the foreseeable future.
But for timestamps that can be used to obtain a time difference between them, TAI should be readily available to be used. Most timestamps are for figuring out time differences here on Earth and not relative to astrological signs, they are for knowing what came before what, etc. They are not for generating human-readable dates. It's silly that such a major use case is hardly implemented on major systems, and instead an unreliable Unix Time is used which can "at any moment" have the same second twice.
[1] https://developers.google.com/time/smear