gistfile1.py

class ElementSerializerMixin(object):
[...]
    creation = serializers.SerializerMethodField('get_creation')
    modification = serializers.SerializerMethodField('get_modification')
    ELEMENT_FIELDS = ('creation', 'modification',)

    def _get_date(self, date):
        return date.strftime('%Y-%m-%dT%H:%M:%SZ')

    def get_creation(self, obj):
        return _get_date(obj.creation)

    def get_modification(self, obj):
        return _get_date(obj.modification)
[...]