The pellets should last indefinitely unless they get wet or damp.
The bags of usually have small perforations to let air out during stacking so the bags don't pop, so even if they haven't been opened they could get wet if they were left outside uncovered.
If they are still solid, go ahead and use them, but don't put any sawdust in the stove. It can't effectively move or burn it.
I also have a Harmon Accentra and I love it. I used about 1.3 Tons this winter for my 1650 sq ft house.