When you're saying Unix time does not include leap seconds, you are making exactly the wrong conclusion. Unix time is not a monotonically increasing number of seconds since the Epoch, because it excludes those seconds which are marked as leap seconds in UTC. I.e. the time between now and the Epoch was larger than the current Unix time shows (by exactly the number of leap seconds in between). See e.g. https://en.wikipedia.org/wiki/Unix_time#Leap_seconds