Have you considered using the Mac Reminders app? Seems ideal for your use case.
At first glance, it would appear that you could use the Periodic trigger, but unfortunately it is designed to operate on specific intervals. For example, if you set it to repeat every 30 minutes, then it triggers only on the hour and half-hour -- not 30 minutes from now.
So, while you might be able to devise a complex set of macros to provide you with a Reminder that is triggered n minutes from now, it doesn't seem worth the effort to me, when the Reminders app will easily do exactly that.
So, here's an example macro and AppleScript to make it easy to set a Reminder: