Re: [LEAPSECS] Fixing POSIX time

From: Markus Kuhn <Markus.Kuhn_at_CL.CAM.AC.UK>
Date: Thu, 19 Jan 2006 19:00:27 +0000

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.

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.

  b) It is a date that very slightly simplifies calendrical calculation,
     because it is located at the end of a leap day and at the end of
     a Gregorian mod 400 years cycle. (Otherwise, you would have to shift
     to that date each time to convert scalar <-> YYYY-MM-DD notation.)

  c) It is a date sufficiently recent, such that implementors will be forced to
     properly test their handling of pre-epoch arithmetic, something
     easily neglected in practice with epochs before 1980.


Markus Kuhn, Computer Laboratory, University of Cambridge || CB3 0FD, Great Britain
Received on Thu Jan 19 2006 - 11:01:06 PST

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