you can use the clock component, but this will work only if your app is still up and running
one method is to convert the current time (method Now) into milliseconds, then compare the milliseconds with the date and time converted in milliseconds you like to send the notifier,,,
see the clock tutorial for examples
it helps to be familiar with Do it to debug your blocks..., see also tip 4 here App Inventor: How to Learn | Pura Vida Apps
see also Live Development, Testing, and Debugging Tools
to send notifications at a specific date and time in the future, use the alarmmanager extension
this also works if the app is in the background or has been closed
Taifun