Sugar doesn't 'go off'. As long as it's not absorbed any moisture (which would simply make it harder to work with), it will be fine. If in doubt though, use it in your tea or coffee and buy a new pack to ice a cake with.
Dried products generally last far longer than their 'best before' dates. I reached for some chicken gravy granules from my kitchen cupboard yesterday and noticed that the 'best before' date was December 2015. The gravy still tasted fine though!