Gas cylinders have a 'test ring' on the cylinder valve neck that indicates that the cylinder has been tested and is safe to use.
As long 'in test' date stamped on the ring is later than the current date, then the cylinder is safe to use.
NB: a cylinder is still safe to use even if the date on the cylinder has passed. However, when this cylinder is returned it will not be refilled until it has been re-tested and a new test ring has been fitted.
Always rotate cylinder stocks to cylinders are always within test date.