Steps to reproduce the issue

Create an admin form with a required date field. I.e. open administrator\components\com_content\models\forms\article.xml and make field "publish_down" required by adding required="true" to field's xml node.
Go to Joomla! administration panel and open an article edit view.
Finish Publishing field should now be a required field.
Add Finish Publishing date, using the date picker of the calendar
Click "Clear" button on the Calendar

Expected result

Some sort of error Message ("This field is required"). No changes to the calendar input field.

Actual result

Article is save.
User is redirected to articles list view
Saved article is checked in

System information (as much as possible)

Joomla! 3.7.5
Php 7.1.7 on local XAMPP (as well as PHP 7.0.22 on productive webserver)
Chrome 60.0.3112.113 (Offizieller Build) (32-Bit)