Because we have to distinguish the two concepts. With timezones we already did it: computers save time (usually) in UTC format, and time is converted to the use

Because we have to distinguish the two concepts. With timezones we already did it: computers save time (usually) in UTC format, and time is converted to the user time zone only when displaying it (at an higher level). This is obviously better, when you save a file for example you don't care about the region of the user, and thus the save timestamp of the file should really be an absolute number, so if you take your laptop and go from one country to another your whole system doesn't break because it sees timestamps in the future, same thing when daylight saving is applied and the clock is brought back/forward by an hour. Leap seconds, if we decide that we should keep them (to me not, because the difference is so little that we would start to notice that in centuries, and who knows not which computer systems we will have but if humanity still exists), should really be handled at time zone level, by shifting of an offset that contemplates leap seconds, and not by slowing up/accelerating clocks.