How can I use a timestamp in user document (Firestore) as a cron time? I need each user to receive points once a month, but each user has a different timestamp.
You can review Schedule functions in order to ensure that your function runs on the correct schedule. This convenient solution builds a Pub/Sub topic and uses Cloud Scheduler to trigger events on that topic.
Also, as a suggestion, I recommend you review the How to schedule a Cloud Function to run in the future with Cloud Tasks (to build a Firestore document TTL) guide written by Doug Stevenson.