[LEAPSECS] choose the form

[Joseph S. Myers]

On Thu, 14 Aug 2008, Steve Allen wrote:


> I also maintain that the "right" forms of the zoneinfo files already

> are proof of concept.  They demonstrate that computers can have an

> uniform underlying time scale while still providing a civil time scale

> that has leap seconds.  The technical issues are basically solved.


As a proof of concept they tell us what *not* to do in any proper solution 
for transmitting both civil time and uniform time.  In particular, do 
*not* change the exported interpretation of past timestamps.  (For 
example, past POSIX timestamps stored in filesystems should remain 
interpreted as UTC according to the POSIX formula.)  As I said in my 
message of 11 February:

  The question does not relate to the *correctness* of those timestamps, 
  it's a case where consistency is much more important than correctness.

  If a system has a timestamp and exports an interpretation of that 
  timestamp, and another system stores that interpretation or a second 
  timestamp derived from it, and then one system changes its interpretation, 
  the timestamps are no longer in sync between the two systems.  This can 
  cause issues such as unnecessary remirroring of very large amounts of data 
  whose timestamps no longer match at the two ends.  See
  http://lists.debian.org/debian-user/1997/03/msg00075.html
  for an example of this from when Debian made the mistake of trying to go 
  it alone with a different interpretation of timestamps.  There are further 
  discussions in the debian-devel archives from March-May 1997 of things 
  that broke.

  If you do not change the timestamp interpretation formula in POSIX, but 
  stop inserting leap seconds at some point (or define that timestamps after 
  that point are in TI not UTC), you can avoid the issue of reinterpreting 
  existing timestamps.

So maybe something like "right", but only with a smooth transition from 
UTC to uniform time at some point well in the future.

-- 
Joseph S. Myers
jsm at polyomino.org.uk