The problem:
In using britishairways website looking for award availability, I noticed a bug when you try to use the "Change" button after you have done a search, the date selection using the calendar malfunctions switching the date from MM/DD/YY to DD/MM/YY, it then either fails with an error message or shows the results for the wrong date.

This bug doesn't happen when you start with a new query, the date calendar works correctly.

The fix:

You can workaround the bug by entering the date in MM/DD/YY format using the keyboard - ignoring the popup calendar and then press the TAB key and the calendar will disappear and your query will work correctly.

Using the "Change" function, allows you to do a series of queries faster with fewer keystrokes, than starting a new query each time.

I am using this as a US-based British Airways member. Perhaps, european-based users see the date in the DD/MM/YY format as is the custom there.