new_contents: from rest_framework import fields, serializers
  
  from month import Month, models
  
  class MonthField(fields.DateField):
    
    def to_internal_value(self, value):
      if isinstance(value, Month):
          month = value
      elif isinstance(value, datetime.date):
          month = Month.from_date(value)
          if len(str(month.year)) < 4:
              raise serializers.ValidationError(
                  self.error_messages['invalid_year'],
                  code='invalid_year',
                  params={'value': value},
              )
      
    def to_representation(self, value):
      return value.strftime(value)
  
subject: Update serializers.py