First off, I want to express how much I less-than-three Join Avenue. =D
But yeah, don't change the time on your DS or you'll have to wait until midnight passes twice before items are available in their shops again. Found that out the hard way after RNG abusing stuff on Black 1.
Also, I was pleasantly surprised to find out that all the shops can hold week-long promotions: for me it happened as soon as I raised all 8 of my shops to rank 10. During promotions, all the shops' prices are halved and raffles give you 2 prizes per day.