Hello Doreen,
The way qRules works is that you assign a command to the Command field, and then the qRules code inside the form processes that command, performs whatever action it indicates, and if the command is one that has some sort of scalar output value, sets that value to the Result field so you can retrieve it there. The user's guide has more details about all the commands that are available.
Yes, the qRules functionality will stop working when the trial expires. If this seems like a good solution to your task, I hope that you will consider purchasing the full version, which never expires and can be used in an unlimited number of forms. It is true that there are ways to do DateDiff without code, but if you want to add or remove rows in repeating tables, qRules or your own custom code are the only options, and qRules is a great value compared to the time and effort involved in writing your own code.
Jimmy