Tanks are good for 12 years from a stamped date, 7yrs from a date with an S after it, and 5yrs from a date with an E after it. Triangle valve means it has an OPD, and most consumer tanks have to have it to get filled. Whatever you do, get the things filled, because then you get 20lbs, not the 15lbs that most if not all tank exchange places give you.
+1 on getting two tanks so you're never completely out. There are tanks out there that have a reserve switch on them which cuts off flow at 20 percent left or so. You flip a switch and it lets you drain it completely empty. They cost a little bit more, but it is nice to have that early warning.