Bits of pieces of the operating systems might handle leap seconds properly, but it's doubtful that every single component that uses time does the right thing. The last two leap seconds have revealed bugs in the kernel: https://lwn.net/Articles/504744/ for the one in 2015 and https://lwn.net/Articles/648313/ for the one in 2016, and I don't think it's unlikely that the one scheduled for December will reveal another.