I read many, many posts about Unix time and many say that a day is 86400 seconds.

But pages like this talk about leap seconds. And this made me confused. And I read that Unix time is based on UTC, and all I can understand is that the different between UTC and TAI is the leap seconds, so then Unix must support leap, otherwise why not saying based on TAI instead of UTC?

To confuse myself more, I tried to see what Windows does, and the answers I found is that it does not count leap seconds, but it says based on UTC, and again confusing, as why saying UTC if it is not counting leap?

Anyway, I wanted to ask exactly on how should one convert an integer amount of seconds to exact date in Unix?