Some activities can be accurately timed, and some things cannot. For instance, you are in a restaurant having a meal, and you need to get the server's attention. He's nowhere to be found. You take a bite and there he is, asking if everything is all right. "Mpphhlths agrthes" you say, as you try to swallow, and he says "good" and walks away.
That was perfect timing on his part. They teach it at servers' school. Your timing as a diner needs work. You'll never be as good as he is though. He does it for a living.
Cooks can time things. A cherry pie always takes the same time to cook, so if someone asks a cook how long it takes to cook a cherry pie, he has a ready answer. Now suppose there is a mystery to be solved. How long does it take to solve a mystery? How long does it take for a detective to solve a case. How long for a doctor to diagnose and treat a patient? You probably know where I'm going with this. I'm explaining why doctors are always late for your appointment. They schedule for a day of quickies, and all it takes is one complication and everybody waits longer.
Actually I'm easing into why mechanics don't get your car finished as quickly as you want it, or sometimes even when promised. It's also why they are reluctant to estimate a delivery time in the first place, especially if there is some mystery involved with your problem. They can't know how long it will take to solve your mystery. They can only guess, based on past experience. Now consider that they might have a few cars ahead of you, with unknown time periods allocated to each. They might go quickly, they might get involved. Asking for an exact time for starting and finishing your problem is, although very important to you, nearly impossible for them. Asking what time you can come back to get instant service isn't helpful either, because of the uncertain times involved with the cars that remain there. They can't know when each will be done.
Ideally, each car would be left at the shop, in proper order, and the mechanic would move on to the next one as soon as he finishes one. If the cars aren't there, well, it's why doctors overbook. They can't afford to be standing around waiting for someone to come in. New car dealerships tell you to drop it off at 8 a.m., and they'll call you when it's ready. Imagine what a waiting room would look like if all patients came in at 8 and waited to be seen. If you've ever been in an emergency room, you don't have to imagine it. That sort of high-handed treatment by dealerships is why there are independent repair shops. Independents will try to schedule your mystery-free jobs, like oil changes and alignments to exact times, if possible, but mysteries take more time, and need to be left off.
Trying to time your repair job to a shop's slow period is also tricky. For instance, people call and ask how busy we are. How do you measure business? Pounds? Feet and inches? Gallons? Or how about the English words: slammed, in the weeds, backed up, or average. How is this helpful to the customer? Being busy is good for the shop, but bad for the customer, if he's in a hurry. So it's usually a difficult conversation with the customer asking for guarantees the mechanic can't give.
Being accurate with timing reminds me of the airline pilot who announced to the passengers that they had lost all navigation equipment, so they didn't know where they were, but the good news was that they were on schedule and making good time. Relax. I guess timing isn't everything after all.