How frustrating do you get when you are forced to wait for a service technician to show up to do scheduled work?   (I am no different than you in this; I get really ... when someone is late.)  You have already set and confirmed an appointment for a specific day... but you can't get the office to nail down the time.  Have you ever heard someone say...

   " ...Well, sometime today..."   or maybe, "...sometime between 9 and 2..."

   Here's my favorite, "...I can't tell you exactly when, but sometime today..."

I don't know about you, but in my opinion, all of these are unacceptable. 

There is a famous saying that goes something like this:  "...If you are on time, then you are late.... if you are 10 minutes early, then you are on time....".  I take this to heart. 

When we set an appointment, it is for a specific date and time, and in 95% of the cases, we will be early.  Our rule is simply that if we feel we will be later than 15 minutes from the time that we said we would be there, then we will give you a courtesy call to notify you beforehand, and tell you how much longer it will take us to get there.