Butter is considered to be perishable because of the fact that it can go bad, especially if it stored improperly or not used within a certain amount of time. You will be able to check the package which the butter comes in so you can see when it will expire. A lot of people make the all too common mistake of not checking the label on the package their butter comes in and they therefore end up using it after it has gone bad.
By doing this you run the risk of exposure to foreign bacteria such as listeria which can be quite harmful if it is consumed. In order to avoid this you will need to keep your butter in the refrigerator or even the freezer if you are not plan on using it anytime soon. By doing this you will be able to avoid having to it spoil before the expiration date.