[LEAPSECS] internet drafts about zoneinfo (POSIX Time)

[Tim Shepard]

> I've described it to coworkers as "time_t is one second per SI second, 

> steady state."  Leap seconds perturb this steady-state in any number of 

> implemetnation dependent ways:

> 

> (1) Time stops for a second.

> (2) The last second of the day repeats

> (3) The first second of the day repeats

> (4) Time slews from N seconds before midnight to N seconds after midnight

> (5) Nothing happens, until NTP notices the phase error and steers it out

> (6) Nothing happens, and time is wrong.

> 

> But I'm not sure how much of this is actually mandated by the standard, 

> and how much is reading between the lines. :)

> 

> Since the standard doesn't define what happens around leap seconds, all 

> of the above behaviors are conformant.


I think (7) is probably the most common:

(7) Time was wrong (by at least a few seconds) before the leap second,
    and time is wrong after the leap second.    (In this case, leap
    seconds matter not at all.)

I cannot think of a situation where civil time accuracy needs to be
any better than +/- one second.  That is why leap seconds are OK and
not a problem in most all cases, even if there are computers around
that are trying to follow the posix spec.   

If NTP is there and (5) happens, what problem could there possibly be?

If you're doing something technical where precise interval time
measurements need to remain accurate (even over leap second events),
then you need to understand your system better than a POSIX spec could
possibly inform you, and I don't think neglecting to maintain UTC (by
neglecting to insert leap seconds as needed to keep it within +/- one
second of the correct time) would change that.

			-Tim Shepard
			 shep at alum.mit.edu