[ntp:hackers] Leap second list file

David J Taylor david-taylor at blueyonder.co.uk
Mon Jan 16 09:28:45 UTC 2012

> You're both right.  Leap-add and leap-del LI values do indicate a leap
> second at the end of the current month, on ntpd's input path.
> However, in deciding whether to indicate a known upcoming leap second
> (however known) to downstream clients, ntpd will not indicate a
> pending leap second in LI until less than one day remains to the
> scheduled insertion/deletion.  So in practice, LI means both at the
> end of this UTC month at the end of today UTC.
> Unless you're using the ACTS modem refclock with a poll interval
> greater than 24h, a day is plenty of notice for the software to do its
> job, though some human operators might appreciate the opportunity to
> notice it sooner.  Given those two factors, I wonder if ntpd should be
> changed to indicate pending leap seconds earlier, perhaps after the
> first hour of the month to guard against accidentally misfiring the
> change a month early.
> Cheers,
> Dave Hart


Whilst not doubting the competence of our coders, a large part of me says 
"If it works, leave it".  After all, changes mean documentation changes as 
well, and make existing copies of documentation false.

Having said that, I can see why you might want more than one day's notice, 
so I would suggest making the notice period seven days, rather than one 
day, if that change can be accommodated.

What would be the danger of firing the leap change seven times, though, 
were this done?  Is your "after the first hour" to avoid the chance of 
accidentally making more than one change per month?  On reflection, 
perhaps the month is the more natural unit in any case.  Make it 24 hours 
so that any time zone doesn't get a potentially confusing indication, 

SatSignal software - quality software written to your requirements
Web:  http://www.satsignal.eu
Email:  david-taylor at blueyonder.co.uk 

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