Windows does not recognize leap seconds (see here). adTempus will execute jobs based on the time indicated by Windows, which may be one second ahead of actual time until your server next synchronizes itself to a time server.

When that happens, adTempus will at worst detect that the system time has moved back by one second, but that won't affect anything because it won't cross a minute boundary, and in any case adTempus by default doesn't rerun jobs for time intervals that get repeated due to clock changes.