[ntp:questions] Re: What went wrong with the leap second

Martin Burnicki martin.burnicki at meinberg.de
Wed Jan 4 10:07:51 UTC 2006


Rob van der Putten wrote:
> Hi there
> Some log entries from various boxes (GMT + 1);

This indicates that either the NTP daemon itself, or its upstream server(s),
may not have received the leap second announcement, and so time
synchronization has been messed up.

Please have also a look at my posting from Jan 3 in the thread with the
subject "Drift handling...".

> Apearently some NTPDs will increase the second length by 1/86400 to avoid
> the leap second. Could this be the cause?

I don't know exacly what you mean here. Normally ntpd just passes the leap
second announcement to the operating system kernel, and the way the leap
second is handled depends on how this is implemented in the OS kernel

> Is there a difference in how GPS and DCF77 handle the leap second?

DCF77 starts announcing the leap second 1 hour before it is inserted. 

The GPS satellites start broadcasting the upcoming leap second shortly after
this has been announced by the IERS. The last leap second has been
announced by the GPS satellites since middle of July, 2005. 

How these announcements are passed to ntpd depends on how the leap second
announcement is handled by the firmware of the DCF77 or GPS receiver, how
the driver module for that particular device evaluates that announcement
and passes it to the ntpd loop filter, and of course by the version of
ntpd, since there have been some modifications across the versions.

> Do some NTP implementations insert the leap second at 23:59:59 local time
> rather then UTC?

Not that I'm aware of. It this has happened this was a bug. 

Normally ntpd works exclusively with UTC, but on the other hand I know there
are people who think this is not what they need, and pass a reference time
to ntpd which is local time rather then UTC. Such an ntpd might distribute
local time rather than UTC, and since ntpd was not aware of this, it would
insert the leap second at what it thinks is UTC midnight, but in fact is
local time midnight.

Maybe our leap second info page is helpful. Please have a look at 

Hope this helps.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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