This has happened to me very often, but only when making a change to dates using the popup calendar after not finding something on initial dates selected.

As my Executive Club account is registered in North America, I'm usually on the US site regardless of where in the world I am (this also makes buying tickets that originate at my home in France tricky, as it throws me an error message saying the search isn't possible unless I log out, change to French site, and start again, but that's a matter for another discussion...)

To me, it appears that everything is coded just fine for the initial search from whichever site, whether US, UK, French or Argentine. When adjusting search dates using the calendar, however, it will show the calendar, I'll click on, say, 12th of April, the field will populate 4/12/2015 automatically as it had before on the US site, and I'll be taken to available flights on 4th of December...

If you're not paying very close attention, it's easy to go through the whole process without noticing, as at no time would I ever have looked at a calendar that far out, and the date format for the initial search on the US site is always formatted by US norms.

My approach has been to avoid using that method for changing dates entirely, or, if searching around a date that's less than a week away, choose 3/3, 4/4, 5/5, etc. so that there's no chance of a mix-up. I can then adjust surrounding dates within that week as I always do when I'm looking over a range of dates.