Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

>>> class S(serializers.Serializer):
...    d = serializers.DateField()

Expected behaviour

>>> S(data={'d': '2018'}).is_valid()
True

Based on the error message:

'Date has wrong format. Use one of these formats instead: YYYY[-MM[-DD]].'

Actual behaviour

>>> S(data={'d': '2018'}).is_valid()
True

Note that optional month/days are not being tested:

I think what I'd actually expect is the current behaviour, in which case it's the error message that's incorrect. But I don't know which is intended.