Backflow preventer tests are due either on the annual anniversay date of original installation or on the date assigned to them by the water purveyor. There is no written rule as to how many days before the due date a backflow preventer can be tested. As an un-written rule it is 30 days but should not be any more than 45 before the due date. Most water purveyors and/or their representatives will work with you if you are trying to make all of your annual tests become due at the same time or if you need to move test dates.