If either the admission stamp or the Form I-94 has a specific date on it, then that is the date that your visa will expire, and you must leave the U.S. However, if you have D/S on either the admission stamp or Form I-94, then you can stay in the U.S. so long as you continue your studies, are an active member of your exchange program or have qualifying employment. It is crucial not to confuse the expiration date with the date on your actual visa. The date on your actual visa is the date you can lawfully enter the U.S. by, not the date you must leave by.