I mean I don't like this. I think Japan is great and I'm an American so this is a little embarassing. But if the Japanese have decided this is good to do, they should have done it yesterday. The next best time would be today.
(I do wonder if it's not quite a bit too late for restrictions like this to be effective. Japan already has 1200 cases, and I have heard they have a US-like approach to testing -- that is, it's shit. Unless they are magically different from every other country, the disease is already spreading in communities there. They will ultimately be forced to take the same steps that all other countries have taken to stop the virus, and that includes controlling the movements not just of foreign nationals/Americans, but also their own citizens, into and out of the country.)