If you race you spend a 'day". If you go for a prace lap, that's another day. Everytime you join an online lobby, that's a day. I see it as, if you have to load a race track, that counts as a day.
LOL 4145 days is over 11 years. GT5 was released in 2010, so they clearly are not calculating your *actual* play time. 4145 days represents the simulated racing career time in GT World Mode, where you have done hot laps/practice laps, A-spec and B-spec race events, license tests, etc., each of which represents one "day" in your simulated racing career.
