leap seconds and the stdc library Post: 94270

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Top Forums UNIX for Advanced & Expert Users leap seconds and the stdc library Post 94270 by Perderabo on Wednesday 28th of December 2005 03:09:56 PM

12-28-2005

Administrator Emeritus

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.

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LEARN ABOUT OPENDARWIN

tcl_gettime

Tcl_GetTime(3)						      Tcl Library Procedures						    Tcl_GetTime(3)

__________________________________________________________________________________________________________________________________________________

NAME

       Tcl_GetTime - get date and time

SYNOPSIS

       #include <tcl.h>

       Tcl_GetTime( timePtr )

ARGUMENTS

       Tcl_Time *   timePtr   (out)	Points to memory in which to store the date and time information.
_________________________________________________________________

DESCRIPTION

       The  Tcl_GetTime function retrieves the current time as a Tcl_Time structure in memory the caller provides.  This structure has the follow-
       ing definition:
	      typedef struct Tcl_Time {
		  long sec;
		  long usec;
	      } Tcl_Time;

       On return, the sec member of the structure is filled in with the number of seconds that have elapsed since the  epoch:  the  epoch  is  the
       point  in time of 00:00 UTC, 1 January 1970.  This number does not count leap seconds - an interval of one day advances it by 86400 seconds
       regardless of whether a leap second has been inserted.

       The usec member of the structure is filled in with the number of microseconds that have elapsed since the start of the second designated by
       sec.  The Tcl library makes every effort to keep this number as precise as possible, subject to the limitations of the computer system.	On
       multiprocessor variants of Windows, this number may be limited to the 10- or 20-ms granularity of the system clock.   (On  single-processor
       Windows systems, the usec field is derived from a performance counter and is highly precise.)

SEE ALSO

       clock

KEYWORDS

       date, time

Tcl									8.4							    Tcl_GetTime(3)