linux-kernel - Re: Bug: Status/Summary of slashdot leap-second crash on new years 2008-2009

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Message-ID: <20090105194720.58406f8a@lxorguk.ukuu.org.uk>
Date:	Mon, 5 Jan 2009 19:47:20 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	David Newall <davidn@...idnewall.com>
Cc:	Chris Adams <cmadams@...aay.net>, linux-kernel@...r.kernel.org
Subject: Re: Bug: Status/Summary of slashdot leap-second crash on new years
 2008-2009

> > For any given time based on the 1970 Epoch there is a single correct
> > answer for the translation between each value and a UTC time.
> 
> This confused me because the sense that I've got from this thread
> suggests otherwise.  Unless I've misunderstood, the time() value for the
> first second of 2009 is one greater than the value for the second to
> last second of 2008 (i.e. 23:59:59), which means that there is no
> translation for the last second.  Put another way, my understanding of
> what's been said is that the epoch is effectively increased by one
> second for each leap second.  Have I got this wrong?

No I should have said from a UTC time to a value, the reverse is slightly
ambiguous - as you say leap seconds cannot be distinguished (well unless
you are using floating point but thats a whole can of worms)

Glibc has /usr/share/zoneinfo/right as well as posix zones which I guess
is Ulrich's vote on the subject.

In a strictly posix environment then for 1003.1 post 2001 the definition
is non-leap seconds since (a notional) 1/1/70 UTC 00:00:00. Including
leap seconds in the definition would have caused problems with existing
date stamps moving them by about half a minute.

The kernel doesn't give a brass monkeys about interpretation on the whole
with one main exception - the CMOS RTC time conversion is done without
factoring in leap seconds.

Alan
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