<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _serialize(self, value, attr, obj):
 """Serialize an ISO8601-formatted date."""

        try:
            return super(DateString, self)._serialize(
                arrow.get(value).date(), attr, obj)
        except ParserError:
            return missing