it was something in the air yesterday.
Almost the same happened to me while booking short, 2 day hop. Just before hitting 'pay now' I noticed that return date is day later so managed to correct is before paying.
those things happen occasionally , once I was asked to drive family of 4 to the airport on their way to Australia. We were almost at the terminal when wife looking at the tickets (remember those ? ) said to husband " John , we are flying tomorrow , not today ". That's only beginning of the story, when they returned home , bit disappointed they found they left back door to the garden wide opened. Sort of happy ending !