I do this all the time: I'll wake up in the middle of the night, look at my clock (which means I pick it up and put it about six inches from my face), and then panic because it says X, where X is a time shortly before or after I'm supposed to be somewhere. And then I cuss up a storm, jump up, run to the bathroom, brush my teeth, and somewhere along the way become aware that it's dark outside. Then I recheck the clock, and the X is 12 hours ahead of the X I thought it was.
I did it last night. I was supposed to meet a couple of friends for lunch today at 12:15pm. I woke up, saw on the clock that it was 12:20, and reached for my phone to call one to say I was on my way. Fortunately, the phone has "am" clearly on it, where I'm bound to see it if I look closely enough to dial. So I didn't actually call either of my friends in the middle of the night.
I don't know why it takes me so long to figure out the time. I don't see well, and I don't have my contacts in during the night, but the window's big and it's pretty obvious if it's dark, but it takes me a lot longer to process than you might think.
The upside is that I'm rarely late for stuff if it's on my mental radar. It's getting things onto the radar that's sometimes hard.
But lunch was lovely today, the food and the company both wonderful.