Every profession measures time differently. Doctors and lawyers measure time in hours or vague lumps. Teachers measure time in minutes, even seconds.
If a doctor (or his office) tell you that something is going to happen "at nine o'clock," that means sometime between 9:30 and Noon. Lawyers, at least in my neck of the woods, can rarely be nailed down to an actual time. Anything that's not a scheduled appointment is "sometime this afternoon." Even a summons to jury duty will list a particular time which just represents the approximate time at which things will start to prepare to begin happening. Further up the Relaxed Time Scale, we find the delivery and installation guys for whom "Between 8 AM and 3 PM Tuesday," means "Not at all on Tuesday."
Meanwhile, in teacher land, 9:00 means exactly 9:00. Other professionals may round off, saying 9:00 when they really mean 8:57 or 9:08, but we mean what we say. If a class starts at 9:00 and ends at 9:51, we are getting our fifty-one minutes of class.
This is one of those things that non-teachers don't entirely get. If you work in an office, time is pretty flexy. If a meeting can benefit by running an extra ten or fifteen minutes, you just do it. But in most schools, when the bell rings, you're done. There is no little extra bit of time you can just throw into the work.
Consequently we tend to measure out our time in coffee spoons. One minute and forty-three seconds left in class? Okay, I can totally get three more practice sentences about participial phrases in before I remind them of tomorrow's assignment. Which is better than realizing that you've got two minutes and twenty seconds left for a three-and-a-half minute piece of business.
Nobody in the business world feels any real difference between a forty-two minute meeting and a fifty-one minute meeting, but most teachers feel a whole world of difference in that nine-minute gap. That's why the phrase "It'll just a take a couple of minutes," doesn't mean a thing to civilians, but makes a teacher's heart sink. I think it's also part of why civilians don't really understand what they're asking when they request or require that teachers add "just one more little" thing to the teaching day.
Other than the fact that it gives some teachers the uncanny ability to act as human egg timers, I'm not sure we benefit much from this heightened time awareness. Yes, teachers learn to be punctual, which is a virtue, I hear. But doctors and lawyers and other folks are fast and loose (well, loose) with time because for them, a task takes as long as it takes, whereas in teaching, a task takes as long as we get to do it.
It's a fantasy of mine to imagine a classroom in which I say, "Okay, class. We're going to work with dependent clauses today, and we're going to keep at it till everyone gets it." I understand that problems that go with that (Mrs. Numberwhacker is up the hall wishing I would be done with clauses so she can get started with quadratic equations), but one of the screwy things about how we're set up in this country is that the most fundamental organizing principle is The Clock. Not the students, teachers, or lessons, but The Clock. I know it's hard to think of another way to manage several hundred humans working on a hundred tasks under one roof, but a guy can dream. And if your school figured out a way to be student or task centered, I'd be fascinated to hear about it.