Shelf Life or Due Date for Testing is mentioned on the cylinder stay plates. Stay plates are the vertical plates you see on the cylinder on which the VP ring is placed. For example in one of those stay plates , if you see D-22 , it implies this cylinder is due for testing in the 4th calendar quarter ( oct-dec)of 2022. New cylinders are tested after 7 years and used ones are due for testing once in 5 years . This is governed by the statutory guidelines of PESO.