There is a difference between the visa expiration date and the length of time you have permission to remain in the U.S.
A visa gives a foreign citizen permission to apply to enter the U.S. at a port-of-entry; it does not by itself permit entry into the U.S. A visa will have both an expiration date and an issuance date and the time in between these dates is the visa validity. As long as the visa is valid you are allowed to travel to a U.S. port-of-entry to seek admission. The visa expiration date does not reflect how long you are authorized to stay in the U.S.
At the port-of-entry, a Customs and Border Patrol (CBP) officer makes the decision to allow you to enter and the officer determines the length of your stay. A CBP officer will give you a small white card, Arrival-Departure Record, Form I-94 or a small green card, Form I-94W for VWP travelers. This card is proof of your lawful admission and length of stay. The CBP officer records either a date or duration of status (D/S) in the lower right hand corner. If your I-94/I-94W contains a specific date that signifies the date you must exit the U.S.
The date or D/S notation, shown on your I-94 /I-94W is the official record of your authorized length of stay in the U.S. and you cannot use the visa expiration date in determining or referring to your permitted length of stay in the U.S. If you overstay the end date of your authorized stay, your visa could be automatically be voided or cancelled.