Warranty is a specific time in which an item is covered.
A Guarantee is nothing really but an exclaimation of validity.
guar·an·tee
n.
1. Something that assures a particular outcome or condition: Lack of interest is a guarantee of failure.
2.
1. A promise or an assurance, especially one given in writing, that attests to the quality or durability of a product or service.
2. A pledge that something will be performed in a specified manner.
3.
1. A guaranty by which one person assumes responsibility for paying another's debts or fulfilling another's responsibilities.
2. A guaranty for the execution, completion, or existence of something.
4. A guarantor.