UTC would save you w.r.t. timezones, but it still has leap seconds and the like to adjust for nonuniformities in the earth’s rotation (around itself and the sun).
However, these jurisdictions change timezones in autumn and spring, so specifying the current timezone should work just as well as specifying UTC. 01:30:20 CEST is not repeated, nor is 01:30:20 CET.
Leap seconds don't cause timestamps to repeat, though. They're done by adding an extra second to a minute, so that instead of going between 0-59 seconds, that one minute goes from 0-60.
