title: Send FCM notification on specific time using firebase cloud function
body: <p>I'm looking for a way to send a firebase cloud messaging notification on a specific time using cloud functions.</p>
  
  <p>At the moment, a notification is sent as soon as a new document is created in a Firestore database, and he time that the notification should be sent on comes from a timestamp in that document.</p>
  
  <p>I have looked at creating a cron job, but I can't find a way to only send the notification once, and not once every day / week. I also looked at using a setTimeout function, but that seems like a bad idea in this scenario  because of the time limit every invocation has.</p>
  
  <p>I have very little experience with both typescript and firebase cloud functions, so i would be glad if someone has an idea on how to do this!</p>