title: Notify user when a date is near
body: <p>I'm currently working on an Android app that uses several Firebase functions. In the Realtime Database I have a date (the date a book is due) and I need to notify the user when that date is near (say, 1 day before the date in the database). Firebase cloud functions doesn't seem to have a specific trigger to do this, as nothing in the database is changing. </p>
  
  <p>I have seen <a href="https://stackoverflow.com/questions/28017943/android-how-to-set-a-notification-to-a-specific-date-in-the-future">this</a> thread, which can set an alarm/notification for a future date, but I don't know how to stop it once it's been set; I need to be able to cancel the notification if they return the book. This does trigger a change in the database, so I would have an event to use to cancel it, if there was a way to do so.</p>
  
  <p>Is this the best way to do this, or is there a way to implement it using Firebase? And if this is the way to go, how would I cancel a scheduled notification?</p>