Cylinders are subject to recertification (also know as requalification) twelve years from their date of manufacture and every five years after that. For example, a cylinder manufactured in January of 2000 will have to be recertified in January of 2012 meaning if you take your bottle to the propane company in April of 2012 to be refilled, it will have to be requalified by authorized personnel before it can be filled.
The recertification process is simple and does not take too long to complete. Think of cylinder recertification as an inspection similar to that of your car. A vehicle has to be inspected annually so that it may continue to operate safely on the road. Similarly, a propane cylinder must be inspected so that it may continue to operate safely.