Saving after the 15 Minutes?

So things I'm seeing online say to wait for 15 minutes at the body disposal thing in the basement, and then to do the conveyor belt part without killing any zombies, instead pushing them into the blades. If I wait the 15 minutes and then save, if I were to restart from that save (due to not actually getting all the zombies in the crusher), do I have to wait the 15 minutes again or will the game remember that I already waited 15 minutes in front of the typewriter? Lol