I've had this issue in the past too. I use the calendar to select a date and sometimes that gets entered in the US format and sometimes in the UK format.
I'm a Brit, in the US, with my computers date/time setting set to UK format (because that's how I read dates) but of course with a US IP address. ba.com gets confused and sometimes sends me to the UK home page and sometimes the US. I've learnt to live with it, and I'm always careful when making any bookings, but there are occassions when the date gets switched and ba.com won't accept the date the other way round and insists that I want the wrong US date instead of the right UK date...