comments on draft ISO C9x changes to <time.h>

[Markus Kuhn]

"Joseph S. Myers" wrote on 1998-06-15 09:29 UTC:
> What is the `correct' time display to give for a leapsecond in a zone with
> an offset from UTC that is not an integral number of minutes?  Have there
> actually been any such zones since the start of the leapsecond system?

There is no official national time zone that is defined as an offset
to UTC but that does not have an integral number of minutes difference
relative to UTC. Obviously, such time zones would be rather difficult to
define, because there would be no obvious notation for the inserted leap
second, as it could not be called 60.

Paul Eggert wrote:

	The tm_zone member is an integer number of minutes.  However,
	common practice (e.g. SunOS 4.x, BSD/OS, Linux) is to have a
	member named tm_gmtoff that is a long number of seconds.  This
	is required for proper support of POSIX.1, which lets the user
	specify UTC offset to the second; it is also required for
	proper support of historical applications.  For example, the
	UTC offset of Liberia was 44 minutes and 30 seconds until May
	1972, and any program running on, say, Linux with the TZ
	environment variable set to "Africa/Monrovia" cannot operate
	correctly with if the UTC offset is required to be a multiple
	of 60 seconds.

I think time zone definitions such as Liberia until May 1972 were obviously
either not based on UTC or were an intellectual error of someone who tried
to define it based on UTC but didn't understand UTC (which is excusable
since at that time it was rather young and as we all know well, people
still have understanding the concept of leap seconds today).

The second offset count would only be useful to allow a good approximation
of time zones that can best be described by an integral second offset
relative to UT0 or UT0, but nobody interested in precision timestamps
would use such a time zone today. People not interested in precision
time stamps do not operate computers with clocks that have an accuracy
of better than 30 seconds, so all this sounds rather academic to me
anyway.

Markus

-- 
Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK
email: mkuhn at acm.org,  home page: <http://www.cl.cam.ac.uk/~mgk25/>