Each time you leave a main menu screen for the track in simulation/GT mode, one game day is added.
This means one day per individual race, one day per individual licence test (you can perform the same test many times in the same session for just 1 game day), one day per series race (each race of a 10 race series would be a day each, but the series race of all 10 is just 1 day), one day per machine test session and one day per Run&Settings session.