Re: [ntpwg] Bug: Status/Summary of slashdot leap-second crash on new years 2008-2009
From: Marshall Eubanks
Date: Thu Jan 08 2009 - 14:57:46 EST
On Jan 8, 2009, at 10:02 AM, M. Warner Losh wrote:
In message: <20090108104854.2dbc41b1@xxxxxxxxxxxxxxxxxxx>
Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> writes:
: > On FreeBSD, Solaris and Digital Unix, I'll point out, that jumping
: > backwards is used, and has been used since at least 1994. So
saying
: > it isn't used in the world today is flat out wrong.
:
: I stand by my comment - when was the last time the IERS used a leap
: second removal ? The code may exist but it doesn't happen.
Jumping backwards is used for every leap second that IERS has ever
done, which was your original comment. There's has never been a case
where there was a leap second for jump forward though. The proper
technical term here is 'negative leap second'. All leap seconds up
until now have been positive leap seconds, and it is unlikely there
ever will be a negative one.
I disagree. In the 1970's, the excess LOD was as much as 3 msec.
After going down some, the mid 1990's it rose to around 2 msec.
Now, it is around 1 msec.
Here is a plot
http://www.iers.org/MainDisp.csl?pid=95-100
Only the long period variations count for leap seconds - the seasonal
and other high frequency
oscillations tend to average out.
In the early part of the last century (~1905), it decreased by ~ 5
msec in a year or so.
If that happened right now, it would go to ~ -4 msec negative, and we
would be seeing
2 negative leap seconds or more per year. Even if the decrease from
1975 to 1985 happened again, it
would be at -1 msec, and we would have a negative leap second every
two years or so.
What is a reasonable assumption is that we would likely have a year or
more warning of the
likelihood of a negative leap second.
Regards
Marshall Eubanks
Warner
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