There are some user interface experts who say that there is no such thing as a user error, only usability errors.

While no system is completely idiot-proof (especially for an idiot this clever), they still could have done a better job. It's good that they highlight in red the non-conforming field, but the error message says "Please enter a valid date", leading the user to conclude incorrectly that the date itself was a problem, not the "Month" field.

They also could have used the international standard format, YYYY-MM-DD.