What is a leap second

Steve Langasek vorlon at dodds.net
Tue Oct 29 19:11:26 CST 2002

On Wed, Oct 30, 2002 at 12:01:11AM +0000, David Laight wrote:
> On Tue, Oct 29, 2002 at 05:50:10PM -0600, Steve Langasek wrote:
> > On Tue, Oct 29, 2002 at 11:44:18PM +0000, David Laight wrote:

> > > What isn't clear (form the associated standards) is what you should do
> > > to the system clock at the point the leap second is added/subtracted.
> > > (Due to variations in the moment of intertia of the earth there
> > > have been seconds added aas well as  subtracted, even though the earth's
> > > rotation is slowing down because the moon keeps stealing angular
> > > momentum from it.)

> > Well, if your system clock keeps proper time as measured in seconds since
> > the epoch in UTC :), you don't need to do anything to the system clock;
> > the leap seconds should then be applied when displaying time in the local
> > time zone.

> No - leap seconds have to be ignored when counting time the epoch.
> Check the posix spec (www.opengroup.org for starters).

POSIX doesn't control the definition of "UTC".  If the system clock is
stored in UTC, then handling of leap seconds is a requirement when
converting from UTC to a local timezone.

Steve Langasek
postmodern programmer

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