old_contents: from datetime import timedelta
  
  from django.utils import timezone
  from django.utils.translation import ugettext_lazy as _
  from rest_framework import serializers
  
  from waldur_core.core.serializers import GenericRelatedField
  from waldur_core.structure.models import Customer, Project
  
  
  class DailyHistoryQuotaSerializer(serializers.Serializer):
      scope = GenericRelatedField(related_models=(Project, Customer))
      quota_names = serializers.ListField(child=serializers.CharField(), required=False)
      start = serializers.DateField(format='%Y-%m-%d', required=False)
      end = serializers.DateField(format='%Y-%m-%d', required=False)
  
      def validate(self, attrs):
          if 'quota_names' not in attrs:
              attrs['quota_names'] = attrs['scope'].get_quotas_names
          if 'end' not in attrs:
              attrs['end'] = timezone.now().date()
          if 'start' not in attrs:
              attrs['start'] = timezone.now().date() - timedelta(days=30)
          if attrs['start'] >= attrs['end']:
              raise serializers.ValidationError(
                  _('Invalid period specified. `start` should be lesser than `end`.')
              )
          return attrs
  
new_contents: from datetime import timedelta
  
  from django.utils import timezone
  from django.utils.translation import ugettext_lazy as _
  from rest_framework import serializers
  
  from waldur_core.core.serializers import GenericRelatedField
  from waldur_core.structure.models import Customer, Project
  
  
  class DailyHistoryQuotaSerializer(serializers.Serializer):
      scope = GenericRelatedField(related_models=(Project, Customer))
      quota_names = serializers.ListField(child=serializers.CharField(), required=False)
      start = serializers.DateField(format='%Y-%m-%d', required=False)
      end = serializers.DateField(format='%Y-%m-%d', required=False)
  
      def validate(self, attrs):
          if 'quota_names' not in attrs:
              attrs['quota_names'] = attrs['scope'].get_quotas_names()
          if 'end' not in attrs:
              attrs['end'] = timezone.now().date()
          if 'start' not in attrs:
              attrs['start'] = timezone.now().date() - timedelta(days=30)
          if attrs['start'] >= attrs['end']:
              raise serializers.ValidationError(
                  _('Invalid period specified. `start` should be lesser than `end`.')
              )
          return attrs
  
subject: Fix typo.