11-28-2017
Quote:
While most clocks derive their time from Coordinated Universal Time (UTC), the atomic clocks on the satellites are set to GPS time (GPST; see the page of United States Naval Observatory). The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections that are periodically added to UTC. GPS time was set to match UTC in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with International Atomic Time (TAI) (TAI – GPS = 19 seconds). Periodic corrections are performed to the on-board clocks to keep them synchronized with ground clocks.
Basically there is a nineteen second difference. And every time a leap second is applied to UTC you have to increment that number: currently 19 seconds, so 19 -> 20.
UTC to GPS Time Correction - Knowledge Base English - QPS Confluence
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TIME(2) Linux Programmer's Manual TIME(2)
NAME
time - get time in seconds
SYNOPSIS
#include <time.h>
time_t time(time_t *t);
DESCRIPTION
time() returns the time as the number of seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
If t is non-NULL, the return value is also stored in the memory pointed to by t.
RETURN VALUE
On success, the value of time in seconds since the Epoch is returned. On error, ((time_t) -1) is returned, and errno is set appropriately.
ERRORS
EFAULT t points outside your accessible address space.
CONFORMING TO
SVr4, 4.3BSD, C89, C99, POSIX.1-2001. POSIX does not specify any error conditions.
NOTES
POSIX.1 defines seconds since the Epoch using a formula that approximates the number of seconds between a specified time and the Epoch.
This formula takes account of the facts that all years that are evenly divisible by 4 are leap years, but years that are evenly divisible
by 100 are not leap years unless they are also evenly divisible by 400, in which case they are leap years. This value is not the same as
the actual number of seconds between the time and the Epoch, because of leap seconds and because system clocks are not required to be syn-
chronized to a standard reference. The intention is that the interpretation of seconds since the Epoch values be consistent; see
POSIX.1-2008 Rationale A.4.15 for further rationale.
SEE ALSO
date(1), gettimeofday(2), ctime(3), ftime(3), time(7)
COLOPHON
This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2011-09-09 TIME(2)