[ntp:questions] Leap second functional question

Unruh unruh-spam at physics.ubc.ca
Mon Feb 18 14:41:38 UTC 2008

David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:

>Dean Weiten wrote:

>> As an example, let's say that there was a leap second to be added on 
>> 2008-02-10 at 23:59:59 (hmm, or is that 2008-02-11 at 00:00:00?).  This 

>It would be added at 2007-12-31T23:59:60 or 2008-06-30T23:59:60.  For a 
>deleted second, 2007-12-31T23:59:59 or 2008-06-30T23:59:59 would be deleted.

He is asking how it is added or subtracted. 
His date of Feb 11 was wrong, but was a trivial part of his question. He
wants to know HOW the second is added or subtracted. 

NTP time is UTC which assumes 86400 seconds in each and every year.
Exactly. Ie, those leap seconds are "forgotten" after they occur. 
Thus UTC does not tell you how many actual seconds occur between date A and
date B/ There is a time scale,  TAI which is the actual number of seconds.
There have apparently been debates as to whether ntp should keep UTC or
TAI. The latter makes more sense. The former is easier for operating
systems to deal with since clocks and legal stuff it UTC (with timezone

AFAIK ntp deletes a leap second by jumping, and inserts a second by
stopping the clock for a second. -- well I am not sure that ntp actually
does that. It may just hand the leap second to the kernel and hope the
kernel does the right thing. If the kernel does not, then ntp spends the
next hour readjusting the clock. 

More information about the questions mailing list