[ntp:questions] Leap second functional question

Unruh unruh-spam at physics.ubc.ca
Mon Feb 18 18:20:56 UTC 2008

"David J Taylor" <david-taylor at blueyonder.neither-this-bit.nor-this-bit.co.uk> writes:

>Unruh wrote:
>> "David J Taylor"
>>> No.  At least on Windows.  IIRC, it doubles the clock speed for one
>>> second to move forward, and halves the clock speed for two seconds
>>> to move backwards.  Something like that.  No step is used.  My
>>> recollection is at best vague about this, but searching the
>>> newsgroup archives may help you.
>> Is this a windows OS dependent procedure? This of course means that
>> the
>> time is wrong for at least two seconds, plus errors due to
>> overshoots, etc.

>You would need to check with the implementors, or back in this newsgroup 
>archives.  It means that any step is avoided which, IMHO, is far more 
>important.  I haven't seen any "errors due to overshoots".

Also a step forward is not a problem. A step backward is a problem and
should always be avoided on a computer.

By overshoot error, I mean that they speed up the clock for one second say.
How do they know when 1 sec has passed? The timer tick could have been
delayed for some reason ( or even lost) suddenly the clock has been running
at double speed or half speed not for 1 sec but for 1.01 sec or 1.05 sec.
That is a large  error ( even on linux with a 100Hz clock it is up to 5ms).
ntp has to try to correct that error. that is what I mean by overshoot.
Note that leap seconds have been happening so rarely that almost noone has
seen one happen, or watched the system as the clock goes through that.

Having ntp run on TAI would certainly be simpler, but would of course make
the time keeping on the system much more complicated.


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