The Alarm Clock makes no sense
It's played for laughs, but it's there; proof that the alarm clock not only makes no sense, but is a really idiotic thing to use for pointing out the moment Marty should 'hit the gas'.
What proof, you ask?
Remember the 'hilarious' scene, where future tech is shown to give an ALERT (important), and the cafe owner reacts with a weird scoff, while Marty tries to hide his..
..DIGITAL WATCH WITH AN ALARM..
..and act all 'casual' about it? Ha ha, such a funny scene.
Why would Doc, who is known to be meticulous and good with math and accuracy of all kinds of things, trust a lousy 'analog' (I know, but what am I gonna say, 'physical'?) alarm clock that doesn't even have a second hand over an accurate DIGITAL CLOCK that Marty has, that probably measures hundredths of seconds, or at least tentsh of seconds easily and accurately?
Why not use Marty's digital wristwatch to signify the correct moment when he's supposed to 'hit the gas', instead of that really clunky and old-looking (even for the 1950s!) alarm clock, whose mechanical timing system simply can't compare to the accuracy of an 1980s digital watch?
The watch is shown to a) work and b) give an alarm (though what was that alarm for? From the watch's point of view, it is the middle of the night, not even CLOSE to morning. Why would Marty program an alarm at that time?).
This means, there is no excuse - with SO MUCH at stake, Doc humorously chooses between an EXTREMELY inaccurate clock and at least relatively accurate (probably more accurate than anything Doc has ever used before) digital watch, the more inaccurate clock!
Doc not only makes that really, really stupid choice signifying that he doesn't care about accuracy (which makes no sense), but Marty also ALLOWS him and doesn't even stop to think that hey, maybe that clumsy-looking dime store clock might not serve as well as his own, trusty wristwatch...
What kind of sense does THAT make?
It doesn't.