Leap seconds are not counted in Unix time. Otherwise days with leap seconds would be counted as having 86401 seconds instead of 86400 seconds. This is not the case. In the example you cite, the Unix time resets to 915148800 after the leap second (1999-01-01T00:00:00), so the leap second isn't counted. Unix time differs from the true number of seconds since the epoch by the number of leap seconds (plus the UTC-TAI time difference between 1970 and 1972, before leap seconds were introduced).