Hi:
qRules installs with a user guide and a sample form that can help you. Additional documentation is available here.
For the DateDiff command, the first date parameter is going to be the one you are subtracting the second date from. So, if /date1 is 2010-01-10 and /date2 is 2010-01-01, and /component is days, you'd expect to see a result of 10. If you switched those, you'd have -10. So, I think in your command, you may have used your end date for date2 instead of date1.