Oxarc explained the whole bottle daring/rental thing to me. Showed them some pictures of a set I am looking at, bottles were purchased set. Spoke to the seller, he told me they were last filled 7 or so years ago. Fellow at Oxarc also told me that if they were purchased from them they tend to not worry so much about the date, that even if the date on temperature is 4 or 5 years overdue they will swap them out. They charge $24 for the hydrotest, so even if the bottles are out of date it is much cheaper than getting new bottles.