Re: how do computer people want their time clocked?

From: Paul Eggert <>
Date: Thu, 31 May 2001 18:35:08 -0700 (PDT)

> Date: Thu, 31 May 2001 12:59:41 +0200
> From: "Deckers, Michael" <Michael.Deckers_at_FUJITSU-SIEMENS.COM>

> What you describe may be required by POSIX but it is wrong for UTC: the
> second starting with the marker 23:59:60 is called "leap second" in UTC
> and (more importantly) it belongs to June or December, not to July or
> January.

No, ITU-R TF.460-5 (the current standard for UTC) allows you to
represent a UTC value using either a count of seconds or a broken-down time.
The body of the standard refers to UTC as a count of seconds in
several places: it talks about UT1-UTC, UT2-UTC, and UTC+DUT1, and all
of these expressions make sense only if UTC is a count of seconds.
For example, see Annex 1.D, which defines DUT1 = UT1-UTC.

Broken-down times like 23:59:60 are defined only on the last page of
the standard, as a way of disambiguating the otherwise-ambiguous UTC
counts of seconds. In some sense these broken-down times are
second-class citizens (no pun intended :-).

> See also [] for the
> dates during the last leap second.

That announcement uses both the second-marker and the count-of-seconds
terminologies. Here is the count-of-seconds part:

  The difference between UTC and the International Atomic Time TAI is:
  from 1997 July 1, 0h UTC, to 1999 January 1, 0h UTC : UTC-TAI = - 31s
  from 1999 January 1, 0h UTC, until further notice : UTC-TAI = - 32s

If we use a 86,400 second-per-day clock a la POSIX, then this part of
the announcement means that on 1999-01-01 our clock jumped backward
from 00:00:01 to 00:00:00.

> I fully agree that 1 d = 86 400 s, no matter which timescale.

More than one ISO standard agrees with you. And yet this statement is
incompatible with the common and natural interpretation of ITU-R
TF.460-5, the interpretation that says that 1998-12-31 was exactly one
day long at Greenwich.

Again, it is this sort of confusion which leads many people to desire
a civil time regime in which every day has 86,400 seconds.
Received on Thu May 31 2001 - 18:35:15 PDT

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