Messages in this thread | | | From | Marshall Eubanks <> | Subject | Re: [ntpwg] Bug: Status/Summary of slashdot leap-second crash on new years 2008-2009 | Date | Thu, 8 Jan 2009 13:57:24 -0500 |
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On Jan 8, 2009, at 10:02 AM, M. Warner Losh wrote:
> In message: <20090108104854.2dbc41b1@lxorguk.ukuu.org.uk> > Alan Cox <alan@lxorguk.ukuu.org.uk> 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 > _______________________________________________ > ntpwg mailing list > ntpwg@lists.ntp.org > https://lists.ntp.org/mailman/listinfo/ntpwg
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