[LEAPSECS] (no subject)

[Tony Finch]

On Mon, 22 Dec 2008, Steve Allen wrote:

>

> I am aware of the interesting breakages that happened when zoneinfo

> files were retroactively modified to be inconsistent with POSIX.

> Clearly that change cannot be done for past history.


It can't be done for future history either, because it breaks invariants
that are relied on all over the place.


> > There is a pervasive assumption in Unix that midnight UT is when

> > time_t % 86400 == 0.

>

> So if the broadcast time scale were to become TI then these operations

> would be taking place at "atomic day midnight" instead of mean solar

> day midnight.  That's still a legitimate kind of day.


This isn't about scheduling operations, it's about inter-converting
between a count of seconds and broken-down civil time. There are lots of
in-kernel functions and standard data formats (network protocols, file
systems) that rely on the "trivial UT" assumption of 86400 seconds per
day. POSIX time is one example amongst many.


> The argument that "atomic time will drift by no more than a few

> seconds from mean solar during this century has been used by those who

> want to abandon leaps.  In the presence of an internationally approved

> atomic time scale I think that argument goes both ways.  It doesn't

> hurt anybody if certain technical operations drift a few seconds from

> civil wall clock time.


Disseminating a linear timescale instead of UTC doesn't address the
problem if civil time continues to be UTC, because all these systems are
more tightly coupled to civil time than they are to the timescale of their
reference clock(s). Civil time is a pervasive dependency - i.e. lots of
software has built-in assumptions about its structure - whereas time
synchronization is a relatively self-contained function.

Tony.
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