It is hard to know when the Book of Joel was written since the events told about could have happened at any point in time between 850 and 400 BC. Most modern students of the Bible believe the locusts were real insects that destroyed the crops, leaving the people hungry. Others think they are a picture of big armies that will invade the land and destroy. They can also be both things, one past, the other future. Joel calls on the leaders and people to turn from their sins and ask God to forgive them. God promises to do that both at that time and in the future whenever similar punishments are needed.