You say you prefer a standalone solution, but it seems to me that the only sensible solution is one that is configured from the existing database. If your solution requires someone to manually replicate an existing calendar and keep it up to date, that's destined for the trash heap.
In my view the most sensible solution would be to have a PC application which schedules the alarm notifications based on the calendar. Access the calendar however you prefer - this sort of thing is very common and relatively simple to do on a PC (and much, much harder for an Arduino to do). It also gives you an accurate time source, and automatic timezone and DST management, which would be much harder to do on the Arduino. I'd suggest implementing the PC application as a web app, although you could write it as a fat client if you want. I suspect you'll find the web UI easier to develop and easier to use.
The only involvement for the Arduino would be outputting an alarm on command from the central server, and returning whatever form of acknowledgment or feedback that you decided to support.