[ntp:questions] Purpose of a leap second file?

unruh unruh at invalid.ca
Sun Mar 4 04:13:39 UTC 2012

On 2012-03-03, David E. Ross <nobody at nowhere.invalid> wrote:
> On 2/15/12 10:22 PM, A C wrote:
>> Given the recent thread here about the upcoming leap second, I wanted to 
>> know what additional service/value the leap second file provides that 
>> isn't handled by ntpd receiving a message to add or remove a second.  I 
>> understand the generic ntpd documentation includes a flag to add a leap 
>> second so it seems that the leap second file isn't necessary (and that's 
>> also borne out in searches) but it must provide something otherwise it 
>> wouldn't exist.  The documents mention a "graceful" handling, but what 
>> does that mean exactly?
> A leap-second file is useful if you are processing archived time-stamped
> data and need to convert between TAI and UTC.  When a future leap-second
> is announced and added to such a file, it is also useful when projecting
> future time-dependent events.

It is also useful if you are not running ntp, or if your time source
does not know about leap seconds, or ...
It is also useful in setting up the "real" files for computers in which
the harware clock is kept on TAI and the timezone file is used to
convert to UTC/local time, including leap seconds. 

> In certain situations, the processing of time-stamped or time-dependent
> data requires uniform time intervals.  That is, all minutes must be
> exactly 60 seconds in duration; all hours must be exactly 3600 seconds
> in duration; all days must be exactly 86,400 seconds in duration; etc.
> Such data should use TAI, which exhibits uniform intervals.  To adjust
> such data to UTC, a leap-second file is useful.
> Why require a UTC adjustment from TAI?  If the data represent a
> satellite orbiting the earth and the sub-satellite point on the earth's
> surface is needed, the TAI is converted to UTC, which is then converted
> through UT2 to UT1 and possibly to UT0 before obtaining the sidereal
> time.  Sidereal time then gives the current angle of the earth's
> rotation for computing longitude.

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