Freeplay Mode

Start a new game and instead of running up the stairs, kill 600 (or more) zombies. When you have killed 600 (or more) zombies, run up the stairs, go to the security room, then to the rooftop. Don't bother rescuing the two people up there. Go to the Warehouse through the door, not the Elevator. You will then see the "Queen Cutscene". Having done so, you have skipped the "Back Up Brad" mission. You now have three days to do whatever you want, rescue survivors, defeat Psychos and accomplish achievements. Once the three days are up and you have made it to the Heliport before time was up, you will receive the "B" ending.

Or just kill zombies untill 6 PM. i did it with killing 590 something. it is cool though

I usually just wait until 7pm, not needed to kill any zombies what so ever. If it works at 6 then thats good too.