It's not about how recent the change is, but how "useful" it is to people. The definition of second most people know is that it is 1/60 of a minute which is 1/6

It's not about how recent the change is, but how "useful" it is to people. The definition of second most people know is that it is 1/60 of a minute which is 1/60 of a hour which is 1/24 of a day. That's because people can relate to a day but not to the radiation of some atom. Edit: Unless you are a physicist, of course. Then it matters.