[mikeash]

Right, which is why it seems like the obvious thing to use TAI as the underlying base time in a computer system, and have UTC be just another time zone offset on top of that, so you don't have to deal with leap seconds either.

[bashinator]

Ah, I see what you mean - akin to the OS clock being in UTC, while a user sets their account $TZ environment to their local time.

[mikeash]

Exactly. The underlying clock would store actual seconds since 1970, and when you request UTC it would add the current leap second offset. Which is more or less how GPS does it already, for an actual working example.