I'm going fucking crazy

I have issues at any job that I have if I don't have a calendar and write all of my appointments down on it as soon as I'm told when they are. That's easy enough. I have a bigass binder from staples with both a monthly and daily calendar as well as other paperwork I need for work. I shouldn't have any problems, right? WRONG! I write things down on the wrong day. The bad thing is that I *know* that I repeat myself when I'm talking to whomever to make sure I'm getting it down correctly, and somehow I still get stuff on the wrong day. HOW THE FUCK DO I DO THAT? And, more importantly, WTH do I do about it? My work relies on me to help my clients however they need. I'm not going to be very helpful if I always get people's Dr's appointments on the wrong damn day. Blah, if I can't get this straightened out, I may just have to start working somewhere where the work is the same every damn day, like a call center.