Django 序列化问题

django 序列化类中，若涉及到默认值时，在项目启动后，其值不会再修改，如示例中的date_expired字段，即为项目启动时的时间

class RegistionSerializer(serializers.ModelSerializer):
    """注册"""
    phone = serializers.CharField(required=True, allow_blank=False,
                                  allow_null=False)
    smscode = serializers.CharField(
        required=True, write_only=True, help_text='验证码', max_length=6
    )
    password = serializers.CharField(write_only=True)
    created_by = serializers.CharField(default="Registion")
    source = serializers.CharField(default=User.SOURCE_REGISTION)
    date_expired = serializers.DateTimeField(
        default=timezone.now() + timezone.timedelta(days=3))

    class Meta:
        model = User
        fields = [
            'id', 'name', 'username', 'phone', 'smscode', 'password',
            'created_by', 'source', 'date_expired'
        ]
        read_only_fields = ['id']
    
    # 通过再次赋值进行修改
    def validate(self, attrs):
        attrs["date_expired"] = timezone.now() + timezone.timedelta(days=3)
        return attrs