One more humorous statement about adding time to the countdown timer:
In the movie, there are at least two time checks as the Death Star nears its firing position. I measured the actual passage of time:
From "Rebel base one minute and closing." to "Rebel base 30 seconds and closing." took 104 seconds.
From "Rebel base 30 seconds and closing." to "The Death Star has cleared the planet." took 65 seconds.
Where did all that extra time come from????