Clearing a date field manually change is not saved

When editing a contact, if you empty the value of any date field (eg. by selecting with the mouse and clicking "Delete") and then saves changes, the date remains in the database. Is necessary to press the "clear" link in order to work properly and delete the date.

I believe there is no reason to not allow the deletion in both ways. Maybe I'm wrong but in any case, at least, the field should be set to read-only in order to prevent that the user save the contact believing that the date has been really removed.