TAI-based non-POSIX time_t

Markus Kuhn Markus.Kuhn at cl.cam.ac.uk
Wed Apr 14 12:13:06 UTC 2004

"D. J. Bernstein" wrote on 2004-04-13 20:17 UTC:
> Markus Kuhn writes:
> > such things tend to be neglected, and then the local times get derived
> > inaccurately
> How come you aren't screaming about the much larger local-time errors
> that occur when tz updates are neglected?

Reason 1: Local time errors are larger than a few minutes and are therefore so
severe and obvious that every user will scream instantly and therefore things get
fixed rapidly. This is not the case with subminute errors, as they might be
caused by out-of-date leap-second tables. Therefore, the latter worry me
much more.

Reason 2: Local-time errors affect critical long-term state such as
file-system timestamps much less, as these tend to be in UTC. (Just
an hour ago, we discovered a machine that had its default time zone
set to US East Coast local time instead of London local time. We were
able to fix that problem on-the-fly without any disruption, as most
system state remained unaffected. The machine knew UTC accurately
to within 2 ms all the time.)

> Those errors are thousands of times larger!

Exactly. So they are thousands of times less likely to be missed.

> Why aren't you proposing that UNIX time_t be local time?

See above.

> Eventually we'll all have the TAI-to-local-time tables automatically
> updated through the network.

That would be nice and useful, in particular for the people who really need
physical time (computers that control/monitor geophysical or astronomical
instruments, navigations systems, etc.).

It remains to be seen whether that infrastructure will be in place before
ITU eliminates leap seconds from what we call today UTC.


Markus Kuhn, Computer Lab, Univ of Cambridge, GB
http://www.cl.cam.ac.uk/~mgk25/ | __oo_O..O_oo__

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