The fastest and easiest way to advance days that I know of is to go to the license test menu, select test #1, and then without doing the test, press the left D-pad button twice and select "Next Test", keep doing that. It will make you stop at the Coffee Break following test #8, then return to the license menu and start at test #1 again.
Using this method, it not only wont cost you anything, but you can progress the game 9 days about every 1.5 minutes.
So in just an hour you can progress about a year into the game. 👍
I've used this method many times in order to quickly get a specific car from one of the used car lots.