Contrary to what many people believe, the Flux Capacitor is not what makes time travel possible. Nope. My alarm clock is the time machine. No matter what time I go to sleep, my time machine sees to it I get up at the right moment. Usually the same moment every day. If I ever need a few extra minutes in the morning, all I have to do is set the alarm to go off a little early. It’s that simple. This thing actually gives me time.
We give the alarm clock so much grief. Occasionally inflicting mortal force to deal with an interrupting alarm clock. But you know what, these things unceremoniously and without thanks keep our schedules in check. When I yell at it, and wish destruction upon it, it still does its job. 7 minutes later it buzzes again. 7 minutes. Every time.
Doc Brown knew the power of the alarm clock. That’s why he put one on the dashboard to let Mary know when to step on the gas. Marty had to get back home, and that alarm clock was his ticket.