Changing the 3DS / DS clock will stop all time-based events, such as the growth of Berries, for 24 hours. It will not speed up the growth of your Berries, instead it will halt it for a day.
I'm sorry, I completely overlooked the Platinum tag. From what I could find, the time-reset-stall function was introduced in Gen VI, so using the time clock to speed up Berry growth should work. This is derived more from the lack of information rather than the presence of information, so if someone actually has real life experience speeding up Berry growth, please answer :)