Re: [LEAPSECS] Pre-1972 UTC seconds and days

From: Joseph S. Myers <jsm28_at_CAM.AC.UK>
Date: Thu, 26 Jun 2003 13:09:28 +0100 (BST)

On Thu, 26 Jun 2003, Ed Davies wrote:

> Here's the question: was a UTC second the same as an SI second
> so that days were a non-integral number of UTC seconds or was
> the UTC second slightly longer than the SI second?

My understanding is that these were rate adjustments of the UTC clock
(i.e., a variation in the length of the second). If you look in the right
periodicals for the time - I don't have the details to hand - you will
find details of the adjustments being made to particular reference clocks.

> This question is mostly just for curiousity but it is slightly
> relevant when people define time scales supposedly based on an
> epoch such as 1970-01-01T00:00:00Z. More on this later.

I enquired about that issue on the tz list in May 1998; the answer was
that the 1.999918 seconds of changes between 1970 and 1972 are not
accounted for by systems that use that epoch and account for leap seconds
(as opposed to the more usual straight conversion of a UTC timestamp to
seconds since the Epoch by a formula specified by POSIX)

Joseph S. Myers
Received on Thu Jun 26 2003 - 05:19:41 PDT

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