There is nothing in the code that handles leap seconds.
For system time, QlikView and Qlik Sense relies on the operating system - which is the only sensible thing to do. And for dates in a data base, QlikView and Qlik Sense just assumes that these are correct. The date functions (Year(), Month(), etc.) are just transformations of these dates.
Note that neither Excel-like dates nor POSIX time account for leap seconds either.