The clock tai_clock
measures seconds since 1958-01-01 00:00:00
and is offset 10s ahead of UTC at this date.
That is, 1958-01-01 00:00:00 TAI is equivalent to 1957-12-31 23:59:50 UTC.
Leap seconds are not inserted into TAI.
Therefore every time a leap second is inserted into UTC,
UTC shifts another second with respect to TAI.
For example by 2000-01-01 there had been
22 positive and 0 negative leap seconds inserted
so 2000-01-01 00:00:00 UTC is equivalent to 2000-01-01 00:00:32 TAI
(22s plus the initial 10s offset).