On a 'datetime' column, a valid datetime is "2015-04-09"; MySQL automatically converts this to "2015-04-09 00:00:00". The new jQuery data validation (see https://github.com/phpmyadmin/phpmyadmin/pull/1622) prevents entering the shortened form.
We probably should allow insert when using the shortened form here.
Diff:
Fixed with https://github.com/phpmyadmin/phpmyadmin/commit/6a611abf99d69b22a14a3c2c70b5bb188d8aa43f