I did some more additional research on this question and what I found out was that in the US all raw seafood is frozen due to FDA recommendations. The FDA rules are only recommendations, but the restaurants nearly all follow them, because of legal liability. In other words, if somebody got sick and the restaurant did not follow FDA "recommendations" it could open them up to a law suit.
There are two basic types of freezing. In high-end restaurants they use nitrogen flash-freezing, although this style is becoming more common even among common purveyors. In lower end restaurants or supermarkets, the fish is frozen with dry ice down to -5 F or below and kept that way for at least a week. This will kill parasites such as anasikiasis.
Once the fish is deep frozen and sealed from sublimation, it will last essentially forever, so the sushi you are eating in a restaurant might actually be months or even years old. To thaw it, it is soaked in water. Once it is thawed, it has to be prepared immediately.
In Japan freezing is less common and the most high-end restaurants serve fresh fish. This does result in a significant number of cases of parasite problems in humans.