[LEAPSECS] Look Before You Leap ? The Coming Leap Second and AWS | Hacker News
Joseph M Gwinn
gwinn at raytheon.com
Thu May 21 16:09:57 EDT 2015
"LEAPSECS" <leapsecs-bounces at leapsecond.com> wrote on 05/21/2015 08:02:09
> From: "Eric R. Smith" <ersmith at hfx.eastlink.ca>
> To: Leap Second Discussion List <leapsecs at leapsecond.com>
> Date: 05/21/2015 08:01 AM
> Subject: Re: [LEAPSECS] Look Before You Leap ? The Coming Leap
> Second and AWS | Hacker News
> Sent by: "LEAPSECS" <leapsecs-bounces at leapsecond.com>
> On 19/05/15 08:30 PM, Joseph M Gwinn wrote:
> >> From: "Eric R. Smith" <ersmith at hfx.eastlink.ca>
> >> To: Leap Second Discussion List <leapsecs at leapsecond.com>
> >>> True UTC (with leap seconds) didn't cure a problem the committee
> >>> about, and managed to cause problems they did care about. In short,
> > POSIX
> >>> systems have to be able to work in a cave, with no access to the sky
> >>> knowledge of astronomy.
> >> If POSIX time_t were actually a count of SI seconds elapsed since the
> >> epoch, then a machine in a cave (with an accurate enough clock) could
> >> principle maintain correct timestamps. As it stands though, POSIX
> >> cannot be implemented without access to a UTC reference of some kind,
> >> i.e. access to the sky.
> > Well, while POSIX mentions SI seconds, the standard is careful to say
> > these seconds are not exactly SI seconds (because UNIX workstations can
> > have pretty bad clocks). And the standard specifically disclaims being
> > UTC, despite the appearance. Read the standard carefully. It is
> > and designed to support isolated operation.
> I don't have the actual standard in front of me, but have seen claims
> that POSIX time_t is defined (for years after 1970) to be:
> tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 +
> (tm_year-70)*31536000 + ((tm_year-69)/4)*86400 -
> ((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400
> and that "each and every day shall be accounted for by exactly 86400
> seconds". Is this correct? Since the length of the day is not in fact
> exactly 86400 SI seconds, it would follow that a POSIX compliant system
> has to know how many days have elapsed since the epoch, i.e. it needs to
> have some kind of access to the sky. Am I misunderstanding something?
Yes. The actual standard. HTML access is free. <
Look for Seconds Since the Epoch et al in the Rationale volume.
The disclaim of UTC is explicit.
There was a long thread on this on Time Nuts, where I published the details
and links to the actual standard.
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