I manage my Boss's calendar. He has given me permission to his calendar. Today is Oct 27th. I need to setup an appointment on his calendar for Nov 1st. When I do that an save it - many reminders windows open up on my screen. "All my" appointments and tasks between Oct 27 and Nov 1 open up in separate reminder windows. I have to select dismiss on all them. I also notices I have setup a 15 min. reminder for each of my appointments and when the reminder windows open, each one has a 5 min. reminder showing instead of 15 min.

Your reminders are listed in the which is accessible from the View menu. From there you can open, dismiss, or "snooze" multiple reminders with a single click.