It can be somewhat confusing as to when the 30 days starts running. It would be logical if the 30 days started from the time it was mailed or the time it was received. However, it's a little more confusing than that.
The 30 days start at the beginning of the next billing cycle. So if rent is due on the 1st of the month then the tenants will have until the end of the month to stay in the property. For example, if a landlord sends the 30 day notice on the 5th of August. The tenants can stay until September 30th! Make sure you factor this unusual system of counting into your plans.
Lastly, make sure the 30 day notice is sent via certified mail!