[tz] Google public NTP servers to 'smear' leap seconds.

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Mon Dec 5 16:18:03 UTC 2016

On 2016-12-05 04:04, Tony Finch wrote:
> Paul Eggert <eggert at cs.ucla.edu> wrote:
>> With leap smearing, the ideal clock is at most 0.5 s off UTC, as
>> opposed to being at most 1.0 s off with strict POSIX time.
> It's a bit more complicated than that :-)
> There are a number of variations of leap smear. The ones that centre
> the smear interval on the leap second have an up-to-0.5s divergence
> from UTC. The ones that have the whole smear interval before (or
> after) the leap second have an up-to-1s divergence from UTC.
> If I understand the ntpd documentation, they have implemented a 
> configurable smear interval that occurs before the leap second.

Note their provisos not to provide smear on public servers - to avoid 
legal issues; NTP must be rebuilt to support smear and be configured 
with a leap file and smear interval; server and peer times are still 
UTC, only client packets are fudged with the current offset, and they 
contain a refid address of 254. followed by 2 integer bits and 22 
fractional bits of the signed (negative) offset from 0..-1s, calculated 
using the Google cosine formula.

Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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