On 3/28/2017 1:24 PM, Tony Finch wrote:
Joe Touch <touch(_at_)isi(_dot_)edu> wrote:
When UTC adds a leap second, nothing different happens to POSIX time.
Clearly not. Either it jumps backwards at the start of the leap second, or
at the end of the leap second, or it stops for a second, or it runs slow
for some period of time in a controlled manner (e.g. leap smear) or an
uncontrolled manner (NTP swinging around in a wild effort to resync).
POSIX time just keeps ticking.
It is only in relation to UTC that you would notice anything. And it
isn't POSIX that is jumping - it is UTC.
The POSIX formula that specifies the translation from UTC to time_t
implies that it jumps back at the end of the leap second.
http://pubs.opengroup.org/onlinepubs/9699919799.2016edition/basedefs/V1_chap04.html#tag_04_16
That equation is clearly introduced as "A value that approximates..."
Leap seconds and the difference between 86400 seconds/day and SI
definitions are where the approximation errs.
Joe