[ntp:questions] Leap second

Rob van der Putten rob at sput.nl
Wed Feb 8 17:45:07 UTC 2012

Hi there

Edward T. Mischanko wrote:

> Question:  If I'm using a GPS (Garmin 18x LVC) for a reference clock, Is
> the NMEA time stamp already in UTC?

Yes. It does GPS time as well;

> If my time stamp is already UTC, then won’t I double leap if I have a
> leap file specified in my ntp.conf?

No. From the NTPD doc;
If the latest leap is in the past, nothing further is done other than to 
install the TAI offset. If the leap is in the future less than 28 days, 
the leap warning bits are set. If in the future less than 23 hours, the 
kernel is armed to insert one second at the end of the current day. If 
the kernel is enabled, the leap is done automatically at that time;
otherwise, the clock is effectively stopped for one second at the leap. 
Additional details are in the The NTP Timescale and Leap Seconds white 

If none of the above provisions are available, dsependent servers and 
clients tally the leap warning bits of surviving servers and reference 
clocks. When a majority of the survivors show warning, a leap is 
programmed at the end of the current month. During the month and day of 
insertion, they operate as above. In this way the leap is is propagated 
at all dependent servers and clients.

'surviving servers and reference clocks' are those with a '+' or '*' in 
a 'ntpq -p' overview.

Wim T. Schippers for president

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