Hi,
I always carry my crusader cup and canteen when I leave my house for any reason.
Sounds a bit paranoid there but I guess security blankets can take any form and of the things one might compulsively schlep around a canteen is more benign than many others. You could compulsive about dragging around the rotting corpse of your dear aunt.
Most patent water treatments have expiration dates, compounded iodine tablets last for several years, chlorine dioxide tablets might be listed for six months. This assumes you buy them fresh. Be sure to read the directions and check the expiration dates on any you buy.
You might consider something like Polar Pure. This is essentially pure iodine crystals in a small bottle that makes it easy to create and use a saturated solution. Being pure elemental iodine Polar Pure has no expiration date, ever. One small bottle of Polar Pure will treat a very large amount of water.
Also many filters, as long as they remain unused, last a very long time.