The clock
gps_clock measures
seconds since the first Sunday of January, 1980 00:00:00 UTC
. Leap seconds are not inserted into GPS
. Therefore every time a leap second is inserted into UTC,
UTC shifts another second with respect to GPS
. Aside from the offset from
1958y/January/1 to
1980y/January/Sunday[1],
GPS is behind TAI by 19s due to the 10s offset between 1958 and 1970
and the additional 9 leap seconds inserted between 1970 and 1980
.