I've just booked a holiday/vacation starting in... the wrong year! As far as I can work out, the on-line booking mechanism had the September 2018 option right next to the September 2019 option, and I accidentally selected the one for next year. It was only when I had finished the transaction and the page said "only 414 days to go" that I realised what I'd done. Fortunately, I can cancel. The other upside is that anyone asks me what it feels like to be an incompetent, bumbling old idiot, I'll be able to give them some real insight.