[ntp:questions] Re: ntp servers reporting leap second erroneously?

Martin Burnicki martin.burnicki at meinberg.de
Tue Oct 25 13:31:10 UTC 2005

Danny Mayer wrote:
> Martin Burnicki wrote:
>> The article I was referring to can also be found in the NTP questions
>> mailing list archive:
>> http://lists.ntp.isc.org/pipermail/questions/2005-October/007296.html
>> In the log output you can see that the tick adjustment value is
>> temporarily changed from 156250 to 78125, so the system time temporarily
>> increases with half speed only until the time offset introduced by the
>> leap second has been compensated.
> I hope you mean *decrease* with half speed. You are adding a second not
> subtracting it so the clock needs to slow down.

No, there is just some misunderstanding here. What I meant is the following:

Assuming the system clock is really close to UTC before the leap second.
Both increase at a rate of 1 sec per second, which is "normal speed".

If the leap second is inserted into UTC the system clock would normally also
increase by 1 second, so after the leap second the system clock would be 1
second ahead of UTC without correction:

sys clock      UTC           
------------   ------------
23:59:58.000   23:59:58.000
23:59:59.000   23:59:59.000
00:00:00.000  *23:59:60.000 <- leap sec
00:00:01.000   00:00:00.000 <- w/o correction sys clock 1 sec ahead
00:00:02.000   00:00:01.000 
00:00:03.000   00:00:02.000

If we start slowing down the system clock at the beginning of the leap
second then the system clock "waits" for UTC to catch up:

sys clock      UTC           
------------   ------------
23:59:58.000   23:59:58.000
23:59:59.000   23:59:59.000
00:00:00.000  *23:59:60.000 <- leap sec, start correction
00:00:00.500   00:00:00.000 <- sys clock gains 0.5 sec/sec only
00:00:01.000   00:00:01.000 <- UTC caught up, correction finished
00:00:02.000   00:00:02.000
00:00:03.000   00:00:03.000

This means the *rate* of the system clock is decreased to half speed for 2
seconds, but the system clock still *increases* with half speed during this

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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