I need your help in locating a decent app that can combine Calendars and the alerts functionality in the Reminders app, so that I can receive an alarm for certain entries on my calendar without having to duplicate data into both places. 

You’re in luck! It just so happens that the Calendar app (or iCal as it was known before Mountain Lion) will allow you to create alerts on a per-event basis, and send those alerts to your Mac’s screen or iPhone notification at a predefined time.

After creating your event in the Calendar app, double-click your event to show the Info panel. Click the alert drop-down menu to select the type of alert you want to create. You can create a Message, Message with Sound, Email, or Open File.

All-day events cannot specify a time, but unchecking the All-Day checkbox will let you specify hours and minutes before/after to be reminded.

The typical alert is a Message with Sound. This will cause your Mac (or iOS device if you sync Calendar with iCloud) to beep and display a message on the screen. You can have the message displayed on the same day, the day before, x days before, x days after, or on a specific date. If your event is not all-day, then you will be able to select minutes before, hours before, minutes after, or hours after. When you apply the changes, the Calendar app will remind you of the event at the time specified. 

You can create alerts for events in your iPhone’s Calendar app too—no need to duplicate the event in Reminders.

Even if you don’t use Calendar or iCal on your Mac, just on your iPhone, you can set up alerts for your events. When you create a new event, tap the Alert option and set an alert, ranging from the time of the event back to two days before the event. For events already in your calendar, tap one, tap the Edit button at the top right, and tap the Alert section.

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