[time-nuts] June 30 2015 leap second

Mike George mgeorge at tuffmail.us
Sun Jan 11 11:06:19 EST 2015

The referenced article indicates that they apply the smear to their NTP 
servers and allow
the clients to track the servers.  This approach would place some limits 
on the minimum
lead time you would need to implement the smear.

1) you would have to start early enough for the servers to detect the 
change.   Unless
Google has an internal policy to limit maxpoll you would expect the 
clients to be operating
at the default value of 1024 seconds.  They would have to start well 
before 1024 seconds
to allow the clients to detect the change and start tracking.

2) NTP has a maximum slew rate of 500ppm ( 
http://doc.ntp.org/4.1.0/ntpd.htm) .
In practice you wouldn't be able to use this full range because the 
clients would already
have some error in their clocks.

The scary part of the referenced article is in the third to last paragraph:

"and Google engineers developing code don’t have to worry about leap 

That seems like the kind of attitude that would have caused such a mess 
with computer
timekeeping in the first place.

On 1/11/2015 09:31, Tom Van Baak wrote:
> The google code is "lie(t) = (1.0 - cos(pi * t / w)) / 2.0" and they are wise not to publish the actual window value, w. If it were me I'd make it somewhere between a couple of seconds or couple of minutes but I too would not make it a published or hardcoded constant.
> Here's the link again:
> http://googleblog.blogspot.com/2011/09/time-technology-and-leaping-seconds.html

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