Re: [LEAPSECS] Fixing POSIX time

From: M. Warner Losh <imp_at_BSDIMP.COM>
Date: Thu, 19 Jan 2006 12:35:26 -0700 (MST)

In message: <>
            Markus Kuhn <Markus.Kuhn_at_CL.CAM.AC.UK> writes:
: Poul-Henning Kamp wrote on 2006-01-19 17:56 UTC:
: > >For TAI I'd suggest 1958-01-01, when TAI and UT were set nearly together.
: >
: > I chose the time when TAI became constant rate so that
: > all the rubber seconds are confined to negative values.
: Please remember that the TAI second differed noticeably from the SI
: second until about 1998, because black-body radiation shift was not
: taken into account in the definition of TAI before then. Also caesium
: fountains have improved quite a lot shortly before 2000.

According to the web site that I posted
(, this feature
was in both TAI and UTC. From the url:

UTC on 1977-01-01
 Because the rate of TAI was reduced by one part in 1012, the rate
 of UTC was reduced by the same amount. Therefore, before this date UTC
 seconds were shorter than they currently are.
UTC from 1995 through 1998
 In 1995 a CCTF working group determined that the length of TAI
 seconds was longer than the SI second because the clocks contributing
 to TAI were not corrected for the effects of blackbody
 radiation. Over the next three years the frequency of TAI was steered
 to reduce the length of its seconds by about 2 parts in
 1014. Therefore the length of UTC seconds was also reduced. This
 change is evident as the final kink in the plot of TT(BIPM04). is a frequent contributer here, and this looks like
his web site.

I'd be interested in a source that describes these events differently...

: Therefore, if people ask me for my favourite epoch for a new time scale,
: then it is
: 2000-03-01 00:00:00 (preferably UTC, but I would not mind much
: if it were TAI, or even "GPS time")
: This epoch has the following advantages:
: a) It is well after TAI rubber seconds were fixed in ~1998,
: so we know the time of *that* epoch with much greater accuracy than
: any before 1998.

TAI and UTC have ticked at the same rate since 1972. While this rate
has changed twice (by very small amounts, first by 1 part in 10^12 and
then later by 2 parts in 10^14), they have been the same. Prior to
1972 we had both steps in time (on the order of 50ms to 100ms) as well
as TAI and UTC having different notions of the second.

Received on Thu Jan 19 2006 - 11:38:02 PST

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