This code sets the time to 1:30-something and freezes it there when you turn it on, and lets time count down as normal when you turn it off.
If anyone can figure out how to make it so that you only need R+up/down to activate it, instead of L+R, that would be awesome.
Credit for this code goes to Prof. 9 on the GBAtemp Cheats Forums, and AxelRikkuconnection here. All I did was take Prof's code, and figure out what needed to be changed using AxelRikkuconnection's Time stop code to make it work. I didn't do any of the coding myself, I just have good logic and isolation skills. It works on my DS using the english version of the game.