How do you get overtime mode in dead rising 2

To unlock Overtime Mode in Dead Rising 2, you must:

Complete All Cases
Give Katey doses of Zombrex every 24 hours
Give TK Zombrex before the final fight

This will unlock Overtime Mode, which adds an additional block of time to the game (exact amount is still under review, but somewhere between 24 and 36 hours) in which you must retrieve a handful of items for TK and return them to the Arena.