UNIX for Advanced & Expert Users

I understand the NTP protocol, so keeping system time updated is not a problem.

Standard C library routines like localtime() take a number of UTC seconds elapsed since the start of the epoch (Jan 1, 1970). These times in seconds can be a filetime, system time, or some other time in the past or the future.

In order to translate the seconds into years, months, days, etc. correctly the routines have to know about leap seconds. Including the one coming up this Dec 31, there are 23 leap seconds since 1970.

How is this accomplished? -- knowing about newly hatched leap seconds?
BTW HPUX 11.00 C runtine does not do this correction.... VAX C did.

Code:

/* leapsecond.c */
#include <time.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
	time_t when=strtoul(argv[1],(char **)0,10);
	struct tm *tmptr=gmtime(&when);
	
	printf("%s\n",asctime(tmptr));
	return 0;
}

gives this result:
kcsdev:/home/jmcnama> leapsecond 0
Thu Jan  1 00:00:00 1970

kcsdev:/home/jmcnama> leapsecond `echo "365*2*86400"|bc`
Sat Jan  1 00:00:00 1972

kcsdev:/home/jmcnama> leapsecond `echo "365*3*86400"|bc`
Sun Dec 31 00:00:00 1972

kcsdev:/home/jmcnama> leapsecond `echo "(365*3*86400)+86400"|bc`
Mon Jan  1 00:00:00 1973

There were two leap seconds in 1972, the translation does not take leap seconds into account.

No record of leap seconds is kept. So each time a leap second occurs, calculation of past timestamps becomes wrong by one more second. In the unix view of time, there are no leap seconds. Instead of a leap second, an occasional second is twice as long as other seconds. But this means that you can precalculate, for example, the number of seconds in a year. A system that adheres strictly to the leap second paradigm cannot do that since leap seconds cannot be predicted by more than 6 months. Posix discussed leap seconds at length and decided on this approach. The deciding factor was the need to precalculate future timestamps. Another consideration was that non-networked unix systems may not have access to a leap second knowledgable clock.