Re: [LEAPSECS] Ambiguous NTP timestamps near leap second

From: Warner Losh <imp_at_BSDIMP.COM>
Date: Thu, 16 Feb 2006 16:46:54 -0700 (MST)

From: Rob Seaman <seaman_at_noao.edu>
Subject: Re: [LEAPSECS] Ambiguous NTP timestamps near leap second
Date: Thu, 16 Feb 2006 16:30:37 -0700

> On Feb 16, 2006, at 2:06 PM, Markus Kuhn wrote:
>
> > While there is a 24:00:00, there is certainly *no*
> > 24:00:00.000000000001.
> > That would be 00:00:00.000000000001 instead.
>
> Says who? Didn't we just burn a lot of calories discussing whether
> UTC was a real number or a continuous function? Time does not end at
> midnight, rather, a time without a date or a date without a time are
> incomplete. Are we to believe that there is something evil about
> expressing negative time values, too? Just depends on the epoch.
>
> Put it in angle notation. 12 hours UT = 180 degrees = pi radians,
> but also = -180 degrees or -pi radians. So what?
>
> Would think the usual tactics apply: interpret rules as loosely as
> possible on input and as strictly as possible on output.

UTC rules state that the time sequence should be

23:59:59.75
23:59:60.0
23:59:60.25
23:59:60.50
23:59:60.75
00:00:00.00
00:00:00.25

The problem is for time exchanges that cross the 0:00:00.00 boundary.
Inventing a new notation that is not described in the ITU UTC
definition is not helpful, even if lots of other people use it
informally. It will create confusion because it has no precise
definition in the UTC standard (note: NTP specifically states UTC, and
no other standard).

A better wording for the paragraph would be:

        Time exchanges initiated prior to or during a positive leap
        second must complete before the end of that leap second, or be
        discarded.

Or

        Time exchanges may not span the end of a positive leap
        second. If an implementation detects a time exchange that
        would span this time, it must discard it.

Warner
Received on Thu Feb 16 2006 - 15:49:37 PST