If you buy ready ground cinnamon, there may well be an expiry date on the jar. This doesn't mean that cinammon past this date is actually dangerous, but it will lose a lot of strength and flavour over time. With many spices, if they are kept for a long time, without actually becoming harmful they will reach a point where there is no point in using them, and some can even start to taste unpleasant.
Some studies suggest that ground cinammon loses much of its effectiveness after about 6 months, though the recommended shelf life is more like two years. A lot depends on how it is stored: if it is kept in a sealed container in a dark, dry and cool place, with no contact with heat or damp, it will keep fresh for much longer. A cinnamon stick can be kept for longer.
You can find information about storing ground cinammon here; however, for a really fresh taste you should grind your own spices whenever possible.