Just for sake of practice, I made this reminder tool using Python and PyQt4. You can get code here.
It’s very simple ui that would let you set a reminder time and a message to be shown when the notification goes off. You also have an option to set so the notification window to be closed 10 seconds after it pops on, otherwise it’ll stay open till you close it.
The notification window is pretty simple as well. It’ll just pops on and show you the message that you specified.