Rosa draws a parallel between Day Zero in the police academy and the day zero we all face in real life.  Day Zero is the day before Day One at your dream job.  It is the gut check that happens when you are under pressure and you're not sure how badly you really want it.  It is where you get to work through insecurities and decide that you will not quit.  We all have a day zero and it is a day we can prepare for so that, when it happens, we don't just survive it, we leave a mark that removes any doubt that you belong there.  Day Zero cannot take you out, only you can disqualify yourself.