Android alarms work a bit differently to what you are thinking. The basic alarms are set around times, and when the time comes around, the alarm goes off. So as long as it's after 9:01 am when you use this command, the alarm will go off at 9am the next day correctly.

To deal with days the alarms have a 'repeat' option, listing which days the alarm should go off. Google Now can't handle these kinds of alarms yet. If you needed an alarm for a specific day then you'd probably use calendar alarms, however I don't think they're fully linked into Google Now at the moment.

Yeah this is not the case. It just sets it for the next available 9pm.