I have multiple gas cans, so knowing what is in them, and how old it is gets to be very important. For tags, I cut some from a bleach bottle, and zip tie it to the handle on the gas can. When I fill can up, using a Sharpie, I write the date and if it has ethanol in the gas. As an example, today filled a can, labeled it 07-22-19 A. The "A" tells me it's fuel with alcohol in it, which is only used in generators. Be sure and turn tag so written on side faces down away from the sunlight. I always add Staron to gas when filling cans. If it's 2 cycle mix, along with the date, I write the oil to fuel ratio on tag.