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Re: timestamp resolution

From: Joseph M Gwinn
Subject: Re: timestamp resolution
Date: Wed, 9 May 2007 10:30:54 -0400


Hans Aberg <address@hidden> wrote on 05/09/2007 07:51:16 AM:

> I made a post to the moderated Usenet newsgroup sci.astro.research, 
> "Julian day numbers and leap seconds", and there are so many 
> responses, I cannot summarize them here.
> The question of finding a good time suitable for distributed 
> computing as well as in astronomy and sciences, plus syncing it with 
> legal time is very complicated, and probably requires a committee of 
> scientists and experts from a number of fields.
> But one suggestion might using say TAI (International Atomic Time, 
> with letters in French grammatical order), or possibly GPS time. It 
> might be synced with say JD counted in seconds so that the difference 
> becomes 0 today. Then leap seconds can be then be viewed as a feature 
> of the UTC legal time.

In fact, POSIX "Seconds Since the Epoch" is effectively TAI minus an 
unspecified offset because POSIX counts ~SI seconds regardless of 
astronomy and thus leap anything.  (The fact that ordinary computer clock 
hardware isn't nearly as accurate as that collection of caesium beam 
clocks is neither here nor there - it's the semantics of the timescale, 
not its accuracy, that counts here.)

POSIX time cannot actually be TAI bacause not all POSIX systems have 
access to (or need for) time that's accurate with respect to any external 
timescale.  Think isolated networks with no access to the sky.

As for the choice of the one true clock, the original and still a core 
reason for POSIX to care about time is to support causal ordering of file 
updates by comparison of timestamps. 

The granularity issue has always been with us.  While it is known that no 
finite-resolution timestamp scheme can ensure causal order, the 
alternative (a central guaranteed-sequence hardware utility) is usually 
impractical, so people have always used timestamps.  (IBM sells such a 
utility box for use in their transaction systems.)  What one can most 
easily do is to require much better timestamp resolution as technology 
progresses, thus reducing the window of non-causality in such things as 

The classic reference is:  Leslie Lamport,"Time, Clocks and the Ordering 
of Events in a Distributed System", Communications of the ACM 21, 7 (July 
1978), 558-565.

Lots of other references are under Lamport at <


--- You don't have time to ask me about time ...

> TAI is the mean of highly accurate Cesium atomic clocks (the SI 
> second is defined in terms of Cesium transitions), accuracy better 
> than 10^-7, and is broadcast, so it can be used in computers via 
> radio-controlled clocks (which are now commonly sold). see <http:// 
> en.wikipedia.org/wiki/International_Atomic_Time>.
> As for POSIX, I know that in some quarters, for example NASA when 
> there is a satellite launch, one is very concerned over the problem 
> not knowing which time the computers are set to. So it might be good 
> to fix this gap in the POSIX standard.
>    Hans Aberg

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