The visa is permission to ask to be admitted to the U.S. The visa is valid for the period from the date of issuance until expiration date. As long as the visa is valid, you can apply to be admitted at a port of entry. If your visa has expired prior to your visit to the U.S., you will not be admitted in that visa category. If your visa expires while you are in the US and you want to apply for an extension in order to stay longer, change your status, or get a green card, you may be able to do that. On the other hand, the date on the stamp in your passport or on your I-94, which is a form (mostly electronic now) showing your arrival date and how long you are authorized to stay, shows how long you can stay in the U.S. At the time of admission into the U.S., a border officer determines whether to allow you to enter and for how long you can stay during a particular visit (length of stay), which is reflected on your stamp and/or I-94.