Hi Christine!
Canna seeds will stay viable for years and years even stored in an old dixie cup you left in a drawer in the garage and forgot about 5 years ago.
Canna's are easy to grow from seed but hard to get started because they are round, hard coat seeds and the seed coat has to be nicked which is no easy feat. You can try sand paper, boiling, soaking and a myriad of other ways. Don't waste your time - here is how you do it.
You'll need to break the seed coat before you plant them. The safest way is to hold the seed with a pair of pliers and take a hack saw or dremel tool with a cutting blade. Grab the seed with the pliers and hold on tight enough so that the seed doesn't spin when you saw it. All you want to do is put a slit in the seed coat deep enough to see white. - about 1/16 to 1/8 an inch deep and about half the length of the seed. Once done, plant the seed in good draining potting mix twice as deep as the seed is thick. In a week or two you should see green popping up!
DO NOT UNDER ANY CIRCUMSTANCES TRY TO HOLD THE SEEDS WITH YOUR FINGERS! The seeds are round and slippery and will shoot out of your fingers like a bullet and blood will start leaking out .. I'm serious.