Does cinnamon go bad? I have a small jar of cinnamon and sugar and some of the people in my family are absolutely religious about the best by date.
With spices, that best by date is for flavor, not for safety.
This site states that, under good conditions, you can use cinnamon for 6-12 months after its best before date but notes that, as I've mentioned, it's not harmful, it's due to loss of flavor:
Of course, the shelf life of cinnamon is shorter if it is not stored properly. However, in terms of how long cinnamon lasts, it does not matter too much if it is opened or not as long as it has been properly re-sealed. But remember, like a lot of other spices, it usually has a best before date or a sell by date and not a use by date or expiration date. Because of this distinction, you may safely use it to compliment your favorite foods even after the printed date has lapsed.
Although not a perfect test, your nose is usually the most reliable instrument to tell if it has gone bad. If cinnamon has gone bad, the pleasant aroma will be gone and with it much of the taste. Using this cinnamon will not harm you, but it will not have much flavor. If you choose to use your cinnamon after the eat by date, you can just use a larger quantity to get more flavor.
So, when you let your spices age, they lose their flavor and won't taste as strongly. You can use more to increase the flavor but since you've already mixed it with sugar, that's probably not an option unless you're also going to make it sweeter.
Particularly ground spices will have a relatively short shelf life during which you'll get a good flavor from them. For that reason, I buy many of my spices in small quantities from the bulk section of the store - often only a tablespoon or two at a time. It's cheaper and I get consistently better flavor from them.
Another option is to grind stick cinnamon as you need it but this is not an option I'm personally fond of for cinnamon. I'd rather buy it in smaller amounts and use it quickly. I do use this method for nutmeg, though, which has a very long shelf life when whole but extremely short one when ground.