So on my 3DS, brand new, I haven't changed the time on there. The options for the 3DS itself. However, I've messed with the Animal Crossing time more then once, usually only skipping 3 or 4 days. I can't do the money time travel thing, however. I've tried so many times. What do I do?
To use this cheat/hack, you need to change your 3DS time (in system settings) not the AC:NF time that Isabelle asks if you want to change. It is best to travel as far as you can into the future (the year 2050 I think). Then you will get a letter from the post office saying that you have gained a certain number of bells and they can be taken out at the post office bell point. Something to remember though, you will get lots and lots of weeds throughout your town if you do this multiple times.
NOTE: You only get interest after a month, so a couple of days difference will do nothing.