Is unearned revenue a short term or long term liability?

because it is needed to be earned, that's why it is liability

Let me clarify this answer a bit more. The reason unearned revenue is a liability is because you "owe" something in return for the cash received.

Say a customer pays you for $500 worth of watches and you plan to ship the watches the following month. You have the money for the watches, but you have not supplied the watches yet. Once you supply the watches to the customer, then your unearned revenue is moved to revenue and it is no longer a liability.

Now to the "why". This is because you as a company are obligated to fill the customers order/request to satisfy their purchase. If you are unable to fill the order, then you are liable for the balance and must "pay" the customer back.

So, until the obligation is fully met you must do one of two things,

1. fulfill the obligation with the service or product

2. refund the amount paid in full to the customer

Once one of these two are met, usually the latter, the obligation is met and any money received is "revenue" or money "earned".