Comment by grishka

And even when they let you type it in, sometimes it turns out that the website was made by Americans and so expects the bonkers date format of MM/DD/YYYY.

A good example of appropriate use of a calendar interface on a flight booking website is Aviasales. They show flight prices for each day right there, so if your travel dates are flexible, you know when it's cheaper by just looking at it! This should be a standard feature.