I think you are treating Agenda a bit too much like a task app rather than a note taking app. You can add a date to a note, but it is not really meant as a task to be completed at a specific time. You would be better to use Reminders or a to-do app for that. (We hope to integrate Agenda with Reminders in future)
You can link a note to a calendar event. If you do that, you can have your calendar show an alarm before the event happens. That is probably the best way to go if you want an alarm when the note is about to become relevant.
We hope to improve some of these things in future. Eg. we want to be able to assign a date to a paragraph, and perhaps link a paragraph to a reminder. That will give you much more control.