On his spreadsheet it was on day 389 or something I think. But that doesn't mean it's on your day 389. The car lists are all the same, but they're shifted for different people. It doesn't appear to be tied to PSN, or PS3, or GT5 CD. There's something else generating the starting point but we haven't figured that out yet.
For you to find out where you list is go to your used car dealer and find a rare car in your group of 30. Make note of the cars around it on each side. Then go search his spreadsheet for that rare car. If the cars around it are the same then look at what day that happened on his game. If you then go look at your game day (it's in save data in the XMB) you'll know how many days you're shifted off from him.
You can then use that to predict when a car will show up for you in your game based on when it showed up for him.
Having said all that, you may not find your current list in his list. He only has about 450 days so far, and the cycle appears to be much longer than that, at least 2 game years it appears, so he may not have gotten to the part where your list is yet.
Sorry that's long winded, it's pretty complicated to explain. There's more info in that thread.