[jsmeaton]

How does UTC not save you? There are no DLS transitions in UTC, and leap seconds are added (61 seconds in a minute).

[p4lindromica]

leap seconds may also be negative, although none has been up until this point.

[A1kmm]

Either way you don't get repeated UTC timestamps. A negative leap seconds means you go from 23:59:58 to 00:00:00 without passing 23:59:59, and a positive one means you go from 23:59:59 to 23:59:60 to 00:00:00.