I am thinking about it. We have many time relative processing there, if we want to use the timestamp from database, then we should use it all from database everywhere, otherwise, when the web server is running on a different server from the database one, the timestamp on these machines could be different. There will be unwanted side effect if we reference to different time source. 

At early stage, I want to keep it simple, so just use datetime.utcnow everywhere. After that, when we have different billy web servers, we need to only reference to timestamp from database then.