Time zones are offsets in UTC, which change over time (DST, political changes, etc).
UTC is an offset from TAI, which changes over time (leap seconds).
The time zone files already keep track of historical changes[1].
Conceptually, they're pretty similar; the only difference is that leap seconds have a special clock value (23:59:60 instead of showing you 23:59:59 twice).
I would imagine that some hardware RTCs would not support the :60 leap second value. In fact I cannot recall a single RTC that I have dealt with, that understands leap seconds.
UTC is an offset from TAI, which changes over time (leap seconds).
The time zone files already keep track of historical changes[1].
Conceptually, they're pretty similar; the only difference is that leap seconds have a special clock value (23:59:60 instead of showing you 23:59:59 twice).