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“now” Rule Variable

The now variable contains the number of milliseconds since the UNIX epoch according to the Firebase servers. For instance, you could use this to validate that a user's 'created' time is never set to a time in the future:

{
  "rules": {
    "users": {
      "$user": {
        "created": {
          ".validate": "newData.val() < now"
        }
      }
    }
  }
}