When does the Shampoodle open

You are punished for changing the game time (no matter how much, it seems), but not the system time. Here's what happens:

If you change the time on your 3DS system, Animal Crossing seems to simply think that's the time it is now. Trees grow, events change, the only "bad" things that happen would have happened with the flow of time. Even going back in time via this method seems to work okay, even when going to a day previously played.

If you change the game time, using the option at the main menu of Animal Crossing, you will randomly trip for a while after changing the clock. The duration of tripping seems to last longer than the current play session, possibly the full day? But it eventually goes away. If, however, you were to change the game time this way before every play session you would trip randomly every single session, which is highly frustrating.

Also note that going backwards in time will spoil any turnips you had, since the game knows you went backwards, while it doesn't know you went forwards if you go by the system clock.

tl;dr: Change your 3DS's system time to time travel, do not mess with the in-game clock regularly or will will randomly trip.

http://acwiki3ds.wikispaces.com/Post+Office

I had to check it myself after I got the piggy bank at 1,000,000 Bells instead of the 10,000,000 it was in ACWW.

I can confirm that you receive 0.5% interest on the first of each month (I received 20,000 Bells on 8/1/13 for 4,000,000 in the bank).

Unfortunately I only have 7,000,000 in the bank so I cannot confirm anything past the aluminum case reward (which is indeed at 5,000,000 Bells). What strikes my interest is that you only have to save up 100,000,000 Bells to get all rewards, so it is quite feasible to simply save up that much money at the beginning of the game before doing anything else.

Update: Confirmed all rewards (ADB @ 100,000,000 Bells)