The point about ``connection.commit()`` is interesting indeed, as I'm not doing it now if an errors occurs we might lose all date updates and thus send some emails twice. If we are sure that the database has been updated with the last sending date there is no risk that a user is spamed by us. 

I'll trust you on ``utc_now()`` versus ``run_datetime`` (I actually wanted you to review for this kind of things, I'm not familiar with all the details of crontabber's behavior).