Improvement to calendar in a form.

When you have a calendar field in a form, and you type a date into the field rather than selecting a date from the date picker. If the date you type in isn’t valid e.g. the 30th of February, it will still allow you to submit the form however that data is omitted from the data submitted by the form. Therefore I assume there is a validation which occurs on any calendar to ensure the date is valid and it is omitted where it is not a valid date. It would be great if the validation warned the user their date was invalid when the form is submitted and prevented them from submitting a form with a date that doesn't exist.