Anyway, seeing as how I was tired, somewhat drunk and lightheaded, I was slow to think of a good excuse when my boss called. I should have said, "Sorry, Boss, but I already had plans scheduled," and left it at that. But I actually told the truth and said I hadn't gone to bed yet. That allowed for a few awkward seconds of nervous laughter on both ends -- surely he must have thought, "Who is this freak I hired?" -- and my apologizing, until he accepted the fact that I was an idiot and said he would call someone else.
The more I think about it, it's lame I was asked to come in two hours before I needed to be there. Just call me at midnight if it's absolutely necessary!