I changed my ds clock 3 months back to get some autumn-only items. (I mention this, to exclude the possibility of changing the time one day forward, which might make the game think I already had that day, so I am cheating.)So this is what I noticed: After you change it back, you can't buy anything that day and the day afterwards. (I never set the date of my DS forward, so not to the next day. Yet you still can't buy anything then) You will be able to buy things again the day after that. This is quite irritating though, but I guess it stops abuse by changing the Ds Clock. I have not found a solution to this yet, so I would avoid changing the DS clock altogether. Tell me if you CAN buy items the day after the day you changed the DS clock back. If so, I might be extremely unlucky with my game?
If this is indeed true, I think they wanted to stop people setting their DS clock to 11:59 (just before midnight), so that the date will flip when they just start the game. If so, it seems like they succeeded (unless you go cheating, I think.)