Please show up 15 minutes early

It's annual doctor appointment time for me. 

Yesterday I had a little fun going to my physician's office. I booked the appointment for three o'clock.

A day before the appointment I received a letter in the mail asking me to show up 15 minutes early to my appointment. Why do they ask us to come early? Why not include the 15 minutes into the appointment time? It wasn't a huge deal, but I plan my days around these sorts of things.

I should have considered my observation that doctors' appointments in the afternoon never start close to being on time. There's always something that delays them. I was in the waiting room much longer than 15 minutes after my appointment time.

iPhones are very handy for that sort of thing.