The clock starts ticking on coffee as soon as it's roasted (green beans last a long time). Grinding accelerates the degradation as more of the coffee is exposed

The clock starts ticking on coffee as soon as it's roasted (green beans last a long time). Grinding accelerates the degradation as more of the coffee is exposed to oxygen (nitrogen flushed containers help here). Coffee off its peak isn't harmful, but is less flavorful.