Typically, you must receive the letter within 30 days and the date starts to run the day after you move out. So if you receive the letter on day 31, you need to check your state statutes under the provision of landlord tenant or general provision on times. There should be definition that further explains timing and usually says something like days mean calendar days unless otherwise noted.  Further, the statute will say something like upon receipt unless otherwise noted. So it will mean when it was delivered to you via certified mail, not when you signed for it. If you signed for it a week later, the landlord would be in violation and that is against the public policy of making sure the landlord gives notice, so it is really upon delivery or attempted delivery if you are not home.