Unix -> "include leap seconds and time zone changes" conversion is extremely well studied and well supported. Literally every major operating system does it as

Unix -> "include leap seconds and time zone changes" conversion is extremely well studied and well supported. Literally every major operating system does it as the primary way to display dates - that's what the time-zone database in ICU/CLDR exists to solve. And those changes may affect which "day" you consider something to have occurred on, date alone is frequently not good enough.