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Comment by ratsmack | Hacker News Reader

I have never understood why leap seconds are not phased in over a specific time period rather the current method of adding the entire adjustment at once.

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By making some seconds longer than others? Kind of the opposite of precision timekeeping. Changing the size of the unit of measure is a bad idea compared to changing the number of things measured.

Google shared some details about how they do leap smearing and the adjustments were smeared over a long enough time to be small enough to not affect their applications. They've been doing this for a while with no ill effects AFAIK. I'm aware of other people patching ntpd to do similar stuff as well.

If you can get the smear to be gentle enough to not affect your apps, it's a nice workaround for the general problem. There's just so many components running inside a modern infrastructure, and it's hard to get them all to behave correctly during a traditional 61-second-minute leap second transition.

And that 2012 leap second linux kernel bug was pretty epic. AFAIK, people who are smearing didn't experience it.

http://googleblog.blogspot.com/2011/09/time-technology-and-l...

chrony has support for leap second smearing too. About to turn it on at work to avoid crashing stuff in the lab.

The amount of stuff that still runs an older Linux kernel is insane...

What kind of lab?

Large ISP.

Smearing is not suitable for all applications, however. For example, anything requiring sub-microsecond precision (i.e. data acquisition) would require at least a 12-day smear to not fail its timing requirements.

Such precison is well outside the realm of NTP but is not really a problem for PTP. I've worked on Linux systems that have done this; ~100ns synchronization was not uncommmon.

(PTP, incidentally, got it right and specifies that its timescale is based on TAI, so it is continuous.)

On all hardware components, some seconds are already longer than others. Computer clocks are not accurate. They need to synchronize and the clock skew and the following synchronization cause seconds to have varying duration.

That is exactly how it was done during the 1960'ies: The duration of atomic seconds varied from year to year and it proved utterly unworkable for everybody.

Some NTP servers are configured to do precisely that, in order to avoid the problem entirely in systems where leap seconds might be problematic.

For example, AWS does this for their management console: https://aws.amazon.com/blogs/aws/look-before-you-leap-the-co...

Google also does something similar: http://googleblog.blogspot.com/2011/09/time-technology-and-l...

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