Don't rely on Calendar for reminders; they don't work.

I missed a credit-card payment and was wondering why I hadn't paid. It's because the Calendar alert I set up to remind me every month stopped working.

Alerts are broken on both the computer and iOS when the duration of the event is zero (as a reminder would typically be). On the computer, you might hear the audio alert if you have one set up, but there will be no alert dialog.

On the phone, you may hear part of the audio alert played, but there will be no alert dialog. If you look at your phone, there's no indication of why it made the noise.

Filed a bug on this over six months ago. I encourage you to file your own, since this is a pretty huge defect in a basic application, with the potential to cause you to miss important events.