OK, this was driving me mad. I was trying to do BOS-IAD-LHR-DUB-CPH-LHR-IAD-BOS. Usually not a problem, but the dates kept changing when I went back to the search page. But there is a....

*workaround*: don't input the first date manually when you return to the search page - use the calendar and click on the dates. If you do this, even though the first date changes every time, the rest do not! Saved me a lot of time when I figured this out.