Action Replay Codes:
ACTION REPLAY, US 

NOTE: The following codes work only when used with an 
Action Replay device. These codes WILL NOT WORK without this
device. 
Game ID: AZKE-dd24df4f
There are 2 codes for this game.

Infinite Time (Zoo Keeper/Tokoton/TA Modes) 
92062200 00000202
62062230 00000000
b2062230 00000000
10000000 00007000
10000004 00007000
d2000000 00000000

Infinite Time (Story) 
92062200 00000101
620621f8 00000000
b20621f8 00000000
10000000 00007000
10000004 00007000
d2000000 00000000

Extra Hidden Difficulty Level
To obtain a 4th difficulty, you need to get the highscore on all 4 modes of gameplay:

Zoo keeper - 1200000+ points needed.
Tokoton 100 - 1800000+ points needed.
Quest mode - 10000+ Yen needed.
Time attack - 600000+ points needed.

After getting all 4 high scores, a 4th difficulty appears in the top line of the option menu.

Expert Difficulty
Record the top score in Zoo Keeper, Tokoton 100, Quest and Time Attack modes.