What is a leap second

David Laight david at l8s.co.uk
Tue Oct 29 17:44:18 CST 2002

On Tue, Oct 29, 2002 at 01:22:42PM -0600, Steve Langasek wrote:
> On Tue, Oct 29, 2002 at 06:45:42PM -0000, György 'Nog' Jeney wrote:
> > While I was looking through dlls/ntdll/time.c I came across the following
> > two comments: "FIXME: Compute the number of leap second corrections here"
> > and "FIXME: get the GMT offset here" What do these this mean?
> Just as there are leap years which have an extra day, from time to time
> it's necessary to adjust the clock by adding leap seconds because IIRC, a
> siderial day is not *exactly* 24 hours -- there's a little bit of drift 
> which adds up over time and needs to be corrected for.

I'm fairly sure that the POSIX standard for time ignores leap seconds.
So every calender day has 24 * 60 * 60 seconds (except when the
clock change for summertime).

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.)


David Laight: david at l8s.co.uk

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